Ethanol–ethylene conversion mechanism on hydrogen boride sheets probed by in situ infrared absorption spectroscopy

Abstract

Two-dimensional hydrogen boride (HB) sheets were recently demonstrated to act as a solid acid catalyst in their hydrogen-deficient state. However, both the active sites and the mechanism of the catalytic process require further elucidation. In this study, we analyzed the conversion of ethanol adsorbed on HB sheets under vacuum during heating using in situ Fourier transform infrared (FT-IR) absorption spectroscopy with isotope labelling. Up to 450 K, the FT-IR peak associated with the OH group of the adsorbed ethanol molecule disappeared from the spectrum, which was attributed to a dehydration reaction with a hydrogen atom from the HB sheet, resulting in the formation of an ethyl species. At temperatures above 440 K, the number of BD bonds markedly increased in CD₃CH₂OH, compared to CH₃CD₂OH; the temperature dependence of the formation rate of BD bonds was similar to that of the dehydration reaction rate of ethanol on HB sheets under steady-state conditions. The rate-determining step of the dehydration of ethanol on HB was thus ascribed to the dehydrogenation of the methyl group of the ethyl species on the HB sheets, followed by the immediate desorption of ethylene. These results show that the catalytic ethanol dehydration process on HB involves the hydrogen atoms of the HB sheets. The obtained mechanistic insights are expected to promote the practical application of HB sheets as catalysts.

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1. Introduction

Two-dimensional (2D) materials offer considerable potential for application as catalysts or catalyst supports because of their unique properties such as large surface areas and novel electronic states.^1,2 For example, graphene has shown superior properties as a support,^3,4 while nitrogen-doped graphene exhibits excellent electrocatalytic properties in various reactions.^5,6 2D materials comprising boron are considered to possess unique properties owing to the large number of theoretically predicted polymorphs.^7,8 Various 2D boron-containing compounds, including boron chalcogenides,⁹ boron oxides,^10,11 boron hydrides,^12,13 and boron phosphides,¹⁴ have also been theoretically predicted to have a large number of polymorphs. Depending on the structure, the bonding strengths of the boron materials with gas molecules are expected to be different, thereby leading to a wide variety of catalytic properties for 2D boron-related materials. However, the experimental synthesis of 2D boron-related materials has rarely been reported.^15,16

Hydrogen boride (HB) or hydrogenated borophene sheets are novel 2D boron materials that have been reported recently and are composed of two light elements, namely boron and hydrogen, in a 1 : 1 stoichiometric ratio.¹⁷ HB sheets are terminated by protons on the negatively charged hexagonal boron sheets and they are known to exhibit versatile properties, such as a large H₂ content (8.5 wt%), semimetal electronic properties,^18,19 metal ion reducibility,²⁰ and a light-responsive hydrogen release function.²¹ Our recent study revealed that HB sheets in their hydrogen-deficient state (H : B ratio of approximately 1 : 2.5 ± 0.5) catalyze the conversion of ethanol to ethylene and water above 493 K with high selectivity independent of the contact time, and an apparent activation energy of 102.8 ± 5.5 kJ mol⁻¹.²² The activation energy of this process is comparable to those reported for the catalytic dehydration of ethanol over Al₂O₃ (53–155 kJ mol⁻¹),^23,24 the Lewis acidic catalyst Zr-KIT-6 (79 kJ mol⁻¹),²⁵ silica–alumina (125.5 kJ mol⁻¹),²⁶ and the microporous Fe-ZSM-5 (137.7–271.1 kJ mol⁻¹).²⁷ The formation rate of ethylene on the HB catalyst is lower than that offered by state-of-the-art catalysts, but is of the same order as that on the commercial SynDol (Al₂O₃–MgO/SiO₂) catalyst.²² Therefore, HB sheets can be regarded as a solid-acid catalyst. However, the mechanism of HB catalysis has not yet been elucidated, and the active sites have not been identified. In this study, to gain insights into the HB catalysis of the ethanol conversion process, we conducted in situ Fourier transform infrared (FT-IR) absorption spectroscopy with isotope labelling, since the observation and analysis of the vibrational frequency shifts upon isotope labelling of the intermediate species by in situ FT-IR could lead to determination of the conversion mechanism of the molecules on solid surfaces.²⁸ Overall, we investigate the interactions between the adsorbed ethanol and the HB sheets during heating under vacuum through in situ FT-IR spectroscopy and isotope labelling.

2. Methods

HB sheets were prepared using a previously reported ion-exchange method.^17–22 More specifically, MgB₂ powder (1.0 g, 99%, Rare Metallic Co., Ltd, Tokyo, Japan) in acetonitrile (300 mL, 99.5%, Wako Pure Chemical Industries Ltd, Osaka, Japan) was added to a mixture of an ion-exchange resin (60 mL, Amberlite IR120B hydrogen, Organo Corp., Tokyo, Japan) and acetonitrile (200 mL) in a Schlenk flask under a nitrogen atmosphere. Water was carefully removed due to the facile hydrolysis of MgB₂.²⁹ The resulting mixture was then stirred with a magnetic stirrer at 400 rpm for 2–3 d at room temperature (∼300 K). After this time, the reaction mixture was allowed to settle for a sufficient time and the supernatant was collected and kept for 1 d at 255 K to physically separate the B(OH)₃ byproduct. In samples containing unreacted materials, such as the oxides present in the starting materials, the reaction mixture was filtered through a 1.0 or 0.2 μm pore filter (Omnipore Membrane Filters, Merck Millipore, Billerica, MA, USA) and the filtrate was kept for 1 d at 255 K to physically separate the B(OH)₃ byproduct. Dried HB sheets were prepared by heating the resulting liquid at 343 K, where the gas that evaporated during heating was pumped through a liquid-nitrogen trap. We rigorously characterized the product using X-ray photoelectron spectroscopy (XPS, JPS 9010 TR, JEOL, Ltd, Tokyo, Japan) to confirm the absence of Mg, the presence of negatively charged B, and the absence of oxidized B (a survey X-ray photoelectron spectrum of the HB sheets is shown in Fig. S1, ESI†).^17–22

For in situ FT-IR spectroscopy analysis, we used a homemade vacuum system (Fig. S2, ESI†) coupled with an FT-IR spectrometer (FT/IR-610, Jasco, Ltd, Tokyo, Japan) to observe the structural changes in the HB sheets during heating under vacuum (1.0 × 10⁻⁶ Torr) after exposure to ethanol vapor (∼44 Torr in 123 cc) for 1 h at room temperature (∼300 K). The FT-IR measurements were conducted in transmission mode, with the sample mounted on a CaF₂ disk. Each spectrum was obtained by averaging 20 scans with a resolution of 4 cm⁻¹. The IR absorbance A is defined as A = −ln(T/T₀), where T and T₀ are the transmitted intensities with and without the sample on the CaF₂ disk, respectively. The heating rate was set as 7.2 K min⁻¹; thus, the sample temperature increased by 2.4 K during the measurement time of 20 s for one spectrum (20 scans). The sample temperature was monitored using a K-type thermocouple close to the sample holder. To obtain details regarding the structural changes taking place in ethanol, the isotopes CD₃CH₂OH (ethanol-2,2,2-d₃, Santa Cruz Biotechnology, Inc., Dallas, TX, USA) and CH₃CD₂OH (ethanol-1,1-d₂, Santa Cruz Biotechnology, Inc., Dallas, TX, USA) were used in addition to C₂H₅OH (99.5%, Wako Pure Chemical Industries Ltd, Osaka, Japan). To assign the FT-IR peaks, the vibrational modes of an isolated ethanol molecule were determined using density functional theory (DFT) calculations performed using Gaussian 16.³⁰ The functional and basis set used for the calculations were B3LYP and 6-31++g(d,p), respectively.

To determine the catalytic activity, gaseous ethanol was introduced onto the HB sheets using argon as a carrier gas under atmospheric pressure in a homemade fixed-bed flow reactor, as reported previously.²² The product gas was then analyzed downstream using a gas chromatograph (GC-8A, Shimadzu, Kyoto, Japan) with a thermal conductivity detector equipped with 5A molecular sieves and Porapak Q columns. The catalytic conversion was estimated from the total amount of hydrocarbon produced as follows: ethanol conversion (%) = [(number of carbon atoms in detected hydrocarbon molecules) (mol min⁻¹)/(number of introduced ethanol molecules × 2) (mol min⁻¹)] × 100. The conversion estimated using this method has been confirmed to be similar to that estimated on the basis of the consumption of ethanol under our experimental conditions;²² therefore, the carbon accumulation on HB can be neglected. The W/F conditions (g min mmol⁻¹), where W is the weight of the catalyst (g) and F is the flow rate of C₂H₅OH (mmol min⁻¹), were controlled by adjusting the C₂H₅OH flow rate and the sample weight. Under our experimental conditions (W/F = 4–30), the product exhibited no apparent W/F dependence.²²

3. Results and discussion

Fig. 1 shows the FT-IR spectra of the HB sheets measured under vacuum before and after exposure to ethanol vapor or isotope-labelled ethanol vapor for 1 h at room temperature (∼300 K). In each case, new absorbance peaks originating from ethanol were observed after exposure to ethanol vapor. To assign the absorbance peaks, we compared the peak positions in the experimentally obtained spectra with those from the DFT calculations for isolated ethanol and the reported values for ethanol under various conditions^31–40 as shown in Table 1. In the case of C₂H₅OH (Fig. 1a), absorbance peaks corresponding to the OH group (ν_OH), CH₃ group (ν_CH3), and CH₂ group (ν_CH2) were observed at 3492, 2980, and 2937 cm⁻¹, respectively. An absorbance peak at 2230 cm⁻¹ attributable to the CD₃ group (ν_CD3) was only observed in the case of CD₃CH₂OH (Fig. 1b), and an absorbance peak at 2120 cm⁻¹ attributable to the CD₂ group (ν_CD2) was only observed in the case of CH₃CD₂OH (Fig. 1c). These results suggest that ethanol is adsorbed on the HB sheet at room temperature (∼300 K) under vacuum.

Fig. 1 FT-IR spectra of the HB sheets obtained under vacuum before and after exposure to the vapour of ethanol or isotope-labelled ethanol for 1 h at room temperature (∼300 K); (a) CH₃CH₂OH, (b) CD₃CH₂OH, and (c) CH₃CD₂OH.

Table 1 Experimentally observed absorbance peak positions (cm⁻¹) for ethanol and the isotope-labelled ethanol adsorbed on HB sheets. Peaks were assigned based on comparisons with the DFT (B3-LYP) calculation results for a free ethanol molecule with trans-geometry, experimentally obtained FT-IR spectra for ethanol gas and the isotope-labelled ethanol gas (Fig. S3, ESI), and literature results for ethanol under various conditions.^31–40 Some vibrational modes cannot be clearly assigned unambiguously because of the presence of several modes in the corresponding energy range; in these cases, values are shown in parentheses. Vibrational modes are denoted as follows: ν_s: symmetric stretching, ν_as: asymmetric stretching, ν_ips: in-plane stretching, ν_ops: out-of-plane stretching, and ν_FR: Fermi resonance.³⁹g and t denote gauche- and trans-ethanol, respectively.⁴⁰ Intensity is represented as s (strong), w (weak), vs (very strong), and vw (very weak)³⁹

	CH3CH2OH				CD3CH2OH					CH3CD2OH					Ref.
	ν (OH)	ν as(CH3)	ν s(CH3)	ν s(CH2)	ν (OH)	ν as(CD3)	ν as(CH2)	ν s(CH2)	ν s(CD3)	ν (OH)	ν as(CD2)	ν as(CH3)	ν s(CD2)	ν s(CH3)
On HB	3492	2980	(2937)	(2937)	3499	(2232)	(2964)	(2964)	(2232)	3525	2219	(2975)	2123	(2975)	This work
Gas (Fig. S3, ESI)	3676	2989	(2900)	(2900)	3677	2235	2948	2903	2096	3676	2195	2988	2112	2889	This work
Free molecule B3-LYP trans-geometry	3834	3117	3045	2989	3834	2309	3021	2989	2188	3834	2245	3116	2172	3045	This work
Glass (80 K)	3279	2987	2927	2872	—	2231	2925	2288	2124	—	—	—	—	—	31–33
Multilayer on Si(100) (135 K)	3267	2983	2928	2881	3286	2233	2889	—	2102	—	—	—	—	—	33
Multilayer on HOPG (98 K)	3280	2972	2927	2875, 2896	—	—	—	—	—	—	—	—	—	—	34
Multilayer on CsI (10 K)	3291	2971	2930	2884	—	—	—	—	—	—	—	—	—	—	35
Matrix with H2O on CsI (10 K) CH3CH2OH/H2O = 1/20	—	2978	2904	2871	—	—	—	—	—	—	—	—	—	—	35
Matrix with CO on CsI (10 K) CH3CH2OH/CO = 1/20	3283–3448	2982	2937	2886	—	—	—	—	—	—	—	—	—	—	35
Multilayer on Cu(111) (100 K)	∼3280	2975	2933	2895	—	—	—	—	—	—	—	—	—	—	36
Matrix with Ar (20 K)	3658	2996	2940	2901, 2954	—	—	—	—	—	—	—	—	—	—	37
Free molecule B3-LYP	3808	3106	3035	2987	3808	2297	3088	2987	2182	—	—	—	—	—	33
0.1 M in CCl4	—	—	—	—	—	2229	2928	2884	2123	—	2184–2196	2973	2095–2120	2932	38

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Raman	ν FR(CH2) + νas(CH3)	ν FR(CH3) + νas(CH2)	ν FR(CH3)	ν s(CH3) + νs(CH2)	ν FR(CH2)	ν as(CH2)	ν s(CH2)	ν FR(CH3)	ν as(CH3)	ν FR(CH3)	ν s(CH3)	Ref.
Gas (experiment)	2983	2938	2917	2882	2981	2945	2888	2938	2962–2988	2913	2878	39
Calculation (× 0.973)	2969	2924	2919	2884	2963	2926	2899	2951	3015	2922	2881
Intensity	s	vs	w	vs	s	vw	vs	vs	vw	w	s

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IR and Raman band	ν as(CH3) + νips(CH2)-g	ν s(CH3) + νas(CH2)-t + νFR(CH2)	ν FR(CH3) + νs(CH2)-t + νops(CH2)-g	ν ips(CH2)-g	ν as(CH2)-t + νFR(CH2)	ν s(CH2)-t + νops(CH2)-g	ν as(CH3)	ν FR(CH3)	ν s(CH3)	Ref.
Gas (theoretical)	2973	2930	2876	2960	2900–2930	2876	2973	2850–2900	2933	40

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With the appearance of the ethanol-related absorbance peaks, the intensity of the broad absorbance peak at ∼1300 cm⁻¹ decreased, while those of the peaks at 1650 and 2500 cm⁻¹ increased. Furthermore, following exposure of the HB sheets to ethanol vapor, the peak at 2500 cm⁻¹ shifted to 2520 cm⁻¹ in each case (Fig. 1). These results indicate that the structure of the HB sheets changes upon the adsorption of ethanol. The absorbance peak at 1300 cm⁻¹ can be assigned to the B–H–B vibrational mode,¹⁹ whereas the peak at 1650 cm⁻¹ can be assigned to the bending vibration of H–O–H and/or the vibration of B–H–B bonds at sites with a large strain in the HB sheet, or at the edge of the HB sheet.^17,19 The absorbance peak at 2500 cm⁻¹ can be attributed to the B–H stretching vibration of isolated BH bonds in the HB sheet.¹⁷ These results suggest that the HB sheets may be distorted by the adsorption of ethanol, and some of the BH bonds in the BHB bonds may dissociate to form isolated BH bonds, as indicated by the increase in intensity and shift of the peak corresponding to isolated BH bonds following ethanol adsorption. The complicated profile in the 1000–1500 cm⁻¹ region is caused by not only this change in the BHB bonds of the HB sheets, but also the appearance of absorbance peaks corresponding to the bending, rocking, twisting, scissoring, and wagging modes of the (normal and/or deuterated) methyl and methylene groups of the adsorbed ethanol and/or the stretching mode of CO in the adsorbed ethanol.^28,31–40 Therefore, we mainly focused on the absorbance peaks outside this overlapped region, although some of the peaks were assigned, as shown in Fig. S4–S6 (ESI†).

Fig. 2a shows the FT-IR spectra of the HB sheets with adsorbed C₂H₅OH (referred to as C₂H₅OH/HB) at 300 and 623 K under vacuum. As shown, the ν_OH, ν_CH3, and ν_CH2 peaks almost disappeared at 623 K, while a decrease in peak intensity was observed for the peak of the B–H stretching vibrational mode at 2520 cm⁻¹. In contrast, the absorbance at 1000–1500 cm⁻¹ (the region including the B–H–B vibrational mode) increased with heating. This tendency, in which the peak intensity of B–H decreases while that of B–H–B increases (Fig. 2a), is opposite to that observed during the ethanol adsorption process (Fig. 1). These results indicate that when heated to 623 K, ethanol is no longer adsorbed on the HB sheets and the original state of the HB sheets may somehow be recovered in terms of the B–H–B bonding configuration. Here, we note that the number of hydrogen atoms within the HB sheets should be lower at 623 K than at 300 K because hydrogen molecules are released during the decomposition of the HB sheets over the wide temperature range of 423–1473 K.^17,19,22 Thus, the decrease in peak intensity at 2520 cm⁻¹ may also be caused by the partial decomposition of the HB sheets.

Fig. 2 In situ FT-IR analysis results of the HB sheets with adsorbed C₂H₅OH (C₂H₅OH/HB) during heating under vacuum. (a) FT-IR spectra of C₂H₅OH/HB at 300 and 623 K under vacuum. (b) Absorbance difference spectra during the heating of C₂H₅OH/HB; the FT-IR spectrum at 300 K was subtracted from the spectrum at each temperature (all spectra before subtraction are shown in Fig. S4a, ESI†). (c) Magnified spectra in the OH group (ν_OH), CH₃ group (ν_CH3), and CH₂ group (ν_CH2 and ν_CH2′) regions (note that ν_CH2′ corresponds to a new peak that appeared during heating). (d) Intensity change of each peak in (c) as a function of temperature; the intensity of the final state is set as zero for each case. (e) Conversion of the ethanol reforming reaction investigated using a fixed-bed flow reactor at W/F = 4.8 g min mmol⁻¹ in the presence of HB sheets without heating pre-treatment. The measurements were conducted sequentially while increasing the temperature from 413 to 575 K. (f) Product selectivity at 573 K for the results in (e).

There are two possible reasons for the disappearance of ethanol from the HB sheets during heating, namely desorption and decomposition followed by desorption. To clarify the details of the process on C₂H₅OH/HB during heating, in situ FT-IR spectra of C₂H₅OH/HB were collected during heating under vacuum. Fig. 2b shows the absorbance difference spectra during heating of the sample, in which the FT-IR spectrum at 300 K was subtracted from the spectrum at each temperature; a negative peak indicates a decrease in absorbance with heating whereas a positive peak indicates an increase (all spectra before subtraction are shown in Fig. S4a, ESI†). Fig. 2c shows the magnified spectra in the ν_OH, ν_CH3, ν_CH2, and ν_CH2′ regions (note that ν_CH2′ is a new peak that appeared during heating). As shown by the intensity profile of each peak as a function of temperature in Fig. 2d, each peak clearly shows a different temperature dependence. More specifically, the ν_OH peak intensity decreases with heating and disappears at 453 K, whereas the ν_CH3 peak intensity decreases slowly up to 523 K. Meanwhile, the ν_CH2′ peak intensity increases initially from 400 to 453 K at the expense of the ν_CH2 peak intensity and then decreases from 503 K to 553 K. These results indicate that the disappearance of ethanol is not due to a simple desorption process but instead due to the conversion of ethanol to other species. Importantly, the results presented in Fig. 2a–d clearly show that the OH group of ethanol disappears first. This observation is consistent with previously reported results for the catalytic process under steady-state conditions, where the HB sheets in the hydrogen-deficient state catalyze the conversion of ethanol to ethylene and water through a dehydration reaction (C₂H₅OH → C₂H₄ + H₂O) at temperatures above 493 K.²²Fig. 2e shows the conversion of the ethanol reforming reaction in the presence of HB sheets without heating pre-treatment, as measured using a fixed-bed flow reactor at W/F = 4.8 g min mmol⁻¹. The measurements were conducted sequentially while increasing the temperature from 413 to 575 K. The product selectivity at 573 K is shown in Fig. 2f. Although these measurement conditions differ from those used for the FT-IR measurements of C₂H₅OH/HB under vacuum, the results are consistent in terms of the disappearance of the OH group of ethanol on the HB sheets. Due to the fact that the ν_CH3 and ν_CH2′ peaks are still observed in the FT-IR spectra after the disappearance of the ν_OH peak (Fig. 2b–d), the intermediate decomposition product of ethanol likely remains on the HB sheets at 453 K.

There are two possible intermediate decomposition products of C₂H₅OH that may generate the ν_CH3 and ν_CH2′ peaks even after the disappearance of ν_OH: (i) ethyl species (CH₃CH₂–) and (ii) ethoxy species (CH₃CH₂O–). Herein, it can be interpreted that the intermediate decomposition product is an ethyl species rather than an ethoxy species, because the ν_OH peak did not reappear upon heating after its complete disappearance at 453 K (Fig. 2). Importantly, for the dehydration reaction (C₂H₅OH → C₂H₄ + H₂O) to occur continuously on the HB sheets as a catalytic conversion, as shown in Fig. 2e and as previously reported,²² the OH group must be formed from the ethoxy species or from the decomposed product of the ethoxy species as an intermediate to form H₂O on HB sheets. Furthermore, a continuous decrease in the peak intensities at 1070–1150 cm⁻¹ was observed up to 453 K, ultimately leading to the saturation of the peak change (Fig. S4b, ESI†). This peak can be assigned to the CO stretching vibrational mode of ethanol (ν_CO).^31–37 If the ethoxy species is formed, the ν_CO peak should be observed at a similar position even at temperatures above 453 K, and its intensity should decrease with temperature. Although ν_CO can be interpreted to be present even above 453 K, the intensity decrease is saturated between 453 and 623 K, as can be inferred from Fig. S4b (ESI†). Therefore, the ethoxy species may not be involved in the reaction, despite possibly being formed on the surface. Upon analysis, the ethanol isotope (CD₃CH₂OH and CH₃CD₂OH) results fail to explain this phenomenon (Fig. S5 and S6, ESI†). To sufficiently elucidate the possibility of ethoxy formation, experiments using isotope-labelled ethanol of C₂H₅OD are required, which will be carried out in our future work. However, the absence of the ν_OH peak at temperatures exceeding 453 K suggests that the intermediate decomposition product of ethanol present at temperatures above 453 K is not the ethoxy species but the ethyl species.

To clarify the conversion process of ethanol adsorbed on the HB sheets more in detail, we conducted the same FT-IR experiments with isotope-labelled ethanol (CD₃CH₂OH and CH₃CD₂OH), as shown in Fig. 3 and 4, respectively (all the original spectra before subtraction are shown in Fig. S5 and S6, ESI†). For both isotopes, the observed trends were the same as those for C₂H₅OH (Fig. 2), i.e., the ν_OH peak intensity decreased with heating and disappeared at ∼440 K, whereas the ν_CD3, ν_CH2, ν_CD2, and ν_CH3 peak intensities decreased slowly up to ∼550 K. Interestingly, in the case of CD₃CH₂OH, an absorbance peak attributable to the B–D stretching vibration (ν_BD) appeared at 1895 cm⁻¹ at ∼440 K (together with the disappearance of ν_OH), and the absorbance intensity increased with temperature (Fig. 3c and e). This observation indicates that the CD₃ group of CD₃CH₂OH is decomposed on HB to form a BD bond after the disappearance of the OH group. In other words, the hydrogen deficient HB sheets may undergo a dehydrogenation reaction with the deuterium atom from the CD₃ group of the CD₃CH₂ species on HB. The product of this reaction, CD₂CH₂, might subsequently be desorbed into the gas phase because both ν_CH2 and ν_CD3 intensities decrease continuously upon heating to 550 K (Fig. 3a–d). Similarly, in the case of CH₃CD₂OH, an absorbance peak appeared at 1895 cm⁻¹; however, the peak intensity was considerably lower than that in the case of CD₃CH₂OH (Fig. 3e and 4e). More specifically, the maximum intensity ratios for ν_OH(MAX)/ν_BD(MAX) were 14.0 for CD₃CH₂OH and 3.4 for CH₃CD₂OH. These results indicate that hydrogen abstraction during the dehydrogenation reaction occurs preferentially from the methyl group rather than the methylene group of the decomposed ethanol.

Fig. 3 In situ FT-IR analysis results of the HB sheets with adsorbed CD₃CH₂OH (CD₃CH₂OH/HB) during heating under vacuum. (a) FT-IR spectra of CD₃CH₂OH/HB at 304 and 573 K under vacuum. (b) Absorbance difference spectra during heating of CD₃CH₂OH/HB; the FT-IR spectrum at 304 K was subtracted from the spectrum at each temperature (all spectra before subtraction are shown in Fig. S5a, ESI†). (c) Magnified spectra in the OH group (ν_OH), CH₂ group (ν_CH2), CD₃ group (ν_CD3), and BD group (ν_BD) regions. (d and e) Intensity change of each peak in (c) as a function of temperature. The intensity of the final state is set to zero in (d), whereas the intensity of the initial state is set to zero in (e).

Fig. 4 In situ FT-IR analysis results of the HB sheets with adsorbed CH₃CD₂OH (CH₃CD₂OH/HB) during heating under vacuum. (a) FT-IR spectra of CH₃CD₂OH/HB at 300 and 573 K under vacuum. (b) Absorbance difference spectra during the heating of CH₃CD₂OH/HB; the FT-IR spectrum at 300 K was subtracted from the spectrum at each temperature (all spectra before subtraction are shown in Fig. S6a, ESI†). (c) Magnified spectra in the OH group (ν_OH), CH₃ group (ν_CH3), CD₂ group (ν_CD2), and BD group (ν_BD) regions. (d and e) Intensity change of each peak in (c) as a function of temperature. The intensity of the final state is set to zero in (d), whereas the intensity of the initial state is set to zero in (e).

Of the several possible dehydration mechanisms available to ethanol on the solid surface,^28,41–46 two involve dehydrogenation of the intermediate species by the atoms of the solid surface. More specifically, in the case of CH₃CD₂OH, dehydrogenation may occur after the hydration reaction via the formation of an ethyl species (CH₃CD₂) bonded to boron as an intermediate (CH₃CD₂–B_(in HB)), followed by dehydrogenation at the CH₃ group with the concerted elimination of C–B and C–H bonds; in this case, the BH bond is expected to form without BD bond formation on the HB sheets (thus BD/BH = 0). Alternatively, it may occur via the formation of a free carbenium ion (carbocation) as a transition state, with or without an intermediate state, CH₃CD₂–B_(in HB), followed by dehydrogenation; in this case, the carbocation formed is converted to a more stable symmetric species, in which the two C atoms are equivalent.²⁸ Thus, all H or D atoms can be considered to have the same probability of remaining in the HB sheets when evolving into ethylene. The expected BD/BH ratio in the product sites for this mechanism is therefore 0.67. Using the experimentally obtained maximum intensity ratios for ν_OH(MAX)/ν_BD(MAX) (14.0 for CD₃CH₂OH and 3.4 for CH₃CD₂OH), the corresponding BD/BH ratio can be evaluated as 3.4/14 = 0.24, which is slightly larger than 0 but smaller than 0.67. If we consider the possible isotope exchange reaction for CH₃CD₂OH (for example, the isotope exchange reaction at the transition state from ethyl species to ethylene), the finite value of the ratio is reasonable; thus, dehydrogenation is suggested to occur at the methyl group rather than the methylene group of the ethyl species, without forming a carbocation. This is consistent with the fact that the positions of the ν_CH3 or ν_CD3 peaks do not shift (Fig. 2c, 3c and 4c), indicating that the bonding configuration of the methyl group is maintained as a part of the ethyl species until the methyl group is cleaved by the dehydrogenation reaction (concerted elimination), rather than the formation of another intermediate by sequential complete bond cleavages such as C–B bond cleavage followed by C–H bond cleavage. Concomitantly, the absorbance peak at 2864 cm⁻¹ (ν_CH2′) observed in Fig. 2c can be assigned to the CH₂ group in the ethyl species (CH₃CH₂–B_(in HB)) generated by the decomposition of CH₃CH₂OH via dehydration on the HB sheets. As such, the ν_CH2′ peak intensity initially increases as the ν_OH intensity decreases, and subsequently, the ν_CH2′ intensity begins to decrease together with the ν_CH3 intensity (Fig. 2d) due to the dehydrogenation of the ethyl species.

Fig. 5 shows the Arrhenius plot of the reaction rate k (left axis), where k = (C₂H₅OH conversion [%])/100 × (C₂H₅OH flux [mol s⁻¹])/(HB amount [mol]) was estimated from the results shown in Fig. 2e. The results for ln(ΔA_BD/Δt) are presented on the right axis to evaluate the formation rate of the BD bond, where ΔA_BD/Δt is the time derivative of the ν_BD intensity (A_BD) for the data shown in Fig. 3e. It should be noted that while the values on the vertical axes are different, the width is the same to facilitate comparison of the slopes. Above 480 K, the slopes are similar between ln k and ln(ΔA_BD/Δt), indicating that the BD bond formation rate determines the reaction rate of the ethanol–ethylene conversion on the HB sheets under our experimental conditions, i.e., the rate-determining step is the dehydrogenation of the intermediate ethyl species (CH₃CH₂–B_(in HB)). It is known that the activation energy for the bond cleavage of CD is higher than that for CH due to the zero-point energy difference; the zero-point energy of CH is approximately 1.36 times higher than that of CD, based on the vibrational energy of a simple classical harmonic oscillator with the same spring constant. The similar slopes of ln k (estimated from CH₃CH₂OH) and ln(ΔA_BD/Δt) (estimated for CD₃CH₂OH) in Fig. 5 indicate similar activation energies, which is consistent with the concerted mechanism (discussed in the previous paragraph), where the cleavage of both the C–B and C–H or C–D bonds proceeds simultaneously. Thus, the contribution of the C–H or C–D bond in the apparent activation energy is less for the concerted mechanism compared to that for the complete cleavage mechanism. As shown in Fig. 5, the slope of ln k is slightly less steep than those reported for the HB samples with heating pre-treatment at 573 K under a flow of Ar for 1 h (left axis)²² and a clear flat region can be seen below 480 K. These results indicate that the catalytic activity changes with heating due to the inevitable hydrogen release from HB sheets in the case of the sample without heating pre-treatment.²² However the differences in slopes are small at higher temperatures (above 480 K), suggesting that the rate-determining step of the ethanol–ethylene conversion is also the dehydrogenation of the intermediate ethyl group (CH₃CH₂–B_(in HB)) for the HB sheets with heating pre-treatment. Since the ν_BD intensity is constant above 523 K (Fig. 3e), it is assumed that the formed BD species is not decomposed and released as HD or D₂ but stays on the HB sheets at least at 523 K under our experimental conditions, i.e., the observed slope for ln(ΔA_BD/Δt) may not include the contribution of their decomposition.

Fig. 5 Arrhenius plot of reaction rate k (left axis). k = (C₂H₅OH conversion [%])/100 × (C₂H₅OH flux [mol s⁻¹])/(HB amount [mol]) was estimated from the results shown in Fig. 2e (W/F = 4.8 g min mmol⁻¹). ln(ΔA_BD/Δt) is plotted against 1/T (T is the temperature) on the right axis to evaluate the formation rate of the BD bond, where ΔA_BD/Δt is the time derivative of ν_BD intensity (A_BD) for the data presented in Fig. 3e. The Arrhenius plots for the HB sheets with heating pre-treatment (at 573 K under Ar flow for 1 h) of the sample (W/F = 6.7, 13.6 and 27.2 g min mmol⁻¹)²² are also shown for comparison (left axis).

Finally, we proposed a mechanism for the ethanol dehydration reaction on HB sheets based on the in situ FT-IR analysis, as shown in Fig. 6 and the following chemical equations, using isotope-labelled CD₃CH₂OH:

CD₃CH₂OH_(g) + BHB_(in HB) → CD₃CH₂OH_(a)⋯B_(in HB) + HB_(in HB) (1)

CD₃CH₂OH_(a)⋯B_(in HB) + H_(in HB) → CD₃CH₂–B_(in HB)′ + H₂O_(g) (2)

CD₃CH₂–B_(in HB)′ + B_(in HB)′′ → _(RDS)CD₂CH_2(g) + D–B_(in HB)′′ (3)

where subscript (a) represents an adsorbate on the HB sheets, (g) represents the gas state, (RDS) represents the rate-determining step, and B, B′, and B′′ represent different B atoms in the HB sheets.

Fig. 6 Schematic reaction mechanism for ethanol dehydration on the HB sheets. (a) HB model structure (note that the model structure shown here is the originally proposed local structure,¹⁷ whereas the real sample structure is not crystalline but amorphous and consists of a distorted hexagonal boron network structure with BH and BHB bonds¹⁹). (b) Schematic reaction mechanism for isotopically labelled ethanol (CD₃CH₂OH) dehydration on the HB sheets derived from the in situ FT-IR results. The adsorption structure of ethanol on the HB sheets is not clear, except for the local conversion of the structure from BHB to BH upon adsorption. Therefore, only one possible adsorption structure is shown in I.

More specifically, at room temperature (∼300 K), ethanol vapor is adsorbed on the HB sheets under vacuum, which breaks some of the BHB bonds to form BH bonds as indicated by Fig. 1 (eqn (1) and Fig. 6bI). The adsorption structure of ethanol on the HB sheets is not clear, except for the local conversion of the structure from BHB to BH upon adsorption. Therefore, only one possible adsorption structure is shown in Fig. 6bI, which is assumed to be similar to the proposed adsorption structure of ethanol on H-ZSM5, in terms of the H–O–H configuration.⁴⁷ Next, upon increasing the temperature, the OH group of ethanol may react with a hydrogen atom of the HB sheet, which results in the dehydration of ethanol and the formation of an ethyl group (CD₃CH₂–B_(in HB)) as indicated in eqn (2) and Fig. 6bII, where C–O bond cleavage and C–B bond formation may occur as a concerted reaction. The reaction may be facilitated by tilting the molecular structure to promote contact between C and B (labeled 3 in Fig. 6a and bII). Meanwhile the bonds between the O and H atoms near the B atoms (labeled 1 and 2 in Fig. 6a and bI) are retained, along with a distortion in the hexagonal boron network. This is merely a hypothesis regarding the possible structures, and further examination is required to ascertain the structural change. Meanwhile, hydrogen molecules are released by the partial decomposition of the HB sheets. Finally, as the rate-determining step, the hydrogen-deficient sites of HB (i.e., boron) abstract deuterium from the methyl group of CD₃CH₂–B_(in HB) in a dehydrogenation reaction, where the concerted elimination of C–B and C–D bonds occurs, and the desorption of CD₂CH₂ follows, as indicated by Fig. 2–5 (eqn (3) and Fig. 6bIII). The observed ethanol–ethylene conversion mechanism on the HB sheets is thus different from the so-called intramolecular dehydration reaction of ethanol.⁴¹ However, the mechanism may be similar to that in the case of the H-form zeolite^28,42 in terms of the role of the Brønsted acidic sites and the mechanism of ethanol dehydration; the lattice oxygen and intermediate C₂H₅–O_(lattice) species in the case of the H-form zeolite^28,42 correspond to the lattice boron and C₂H₅–B_(in HB) species, respectively, in the case of the HB sheets.

4. Conclusions

We studied the conversion process of ethanol adsorbed on HB sheets during heating under vacuum through in situ FT-IR absorption spectroscopy with isotope labelling. Up to a temperature of 450 K, the absorption band of OH in the ethanol adsorbed on HB disappeared, suggesting that the dehydration reaction may occur between the OH group of the adsorbed ethanol molecule and the hydrogen atom from the HB sheet. The decomposed ethanol then forms ethyl species on the HB sheets (CH₃CH₂–B_(in HB)). Subsequently, in the rate-determining step, the dehydrogenation reaction forms a bond between the boron atom and a hydrogen atom from the methyl group of CH₃CH₂–B_(in HB), rather than that of the methylene group, by a concerted elimination of the C–B and C–H bonds in CH₃CH₂–B_(in HB) above 440 K. This is followed by the immediate desorption of the ethylene product. These results indicate that the ethanol dehydration process on HB involves the hydrogen atoms of the HB sheets. The obtained mechanistic insights are expected to aid the design of two-dimensional boron-containing materials as effective catalysts for various reactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS Kakenhi (Grant No. 18K18989, 18H03874, 19H02551, and 19H05046:A01), the MEXT Element Strategy Initiative to Form Core Research Centre (JPMXP0112101001), the Ogasawara Foundation for the Promotion of Science & Engineering, and MHI Innovation Accelerator LLC. We thank Prof. Takaki Kanbara for useful discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cp03079a

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