Effects of acid addition on transfer hydrogenolysis of aromatic ethers in hot-compressed 2-propanol over Ni catalyst

Abstract

The effect of acid additives during transfer hydrogenolysis of aromatic ethers (lignin model compounds) in 2-propanol over a Ni catalyst was investigated. HCl, formic acid, and acetic acid, which have been typically used in organosolv lignin extraction, generally decreased the conversion of aromatic ethers and yields of monomers, indicating an inhibitory effect on transfer hydrogenolysis. Blank experiments demonstrated that acidity (proton release) was the primary reason for the inhibitory effect, but gaseous products from carboxylic acids were also a contributing factor. Comparison of acids revealed that the magnitude of inhibition followed the order HCl > formic acid > acetic acid, which corresponded to the order of their acid strength. Furthermore, the effect was discussed based on the potential proton concentration released from acid additives using their pK_a values.

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1. Introduction

Light aromatics (LAs) such as benzene, toluene, and xylene (BTX), as well as phenol, are among the most important building blocks for current chemical industries. These chemicals have been used for precursors in the production of fuels, plastics, fibers, solvents, pharmaceuticals, dyes, etc., and global production of BTX has reached nearly 100 million tons with continuous market expansion.^1,2 Traditionally, these LAs have been obtained from petroleum, which is rich in aromatic structures.^3,4 However, the geopolitical factors influencing petroleum supply pose risks to LAs production.^5,6 Furthermore, the unsustainable perspectives of petroleum resources are becoming increasingly emphasized.^7–9 Consequently, the production of LAs from renewable and widely available feedstocks is highly desirable.

Lignin, the aromatic fraction in lignocellulose, has emerged as a promising alternative feedstock.^10–13 The potential availability of lignin has been reported to be approximately 300 billion tons, with an annual increase of 20 billion tons through photosynthesis.^14,15 Given its abundant aromatic structure and excellent ubiquity in nature, its utilization as a source of LAs will be crucial in the future. However, lignin is a polymer with a heterogeneous structure and is embedded in a lignocellulose matrix.¹⁶ Therefore, the utilization of lignin necessitates two major steps: its fractionation from lignocellulose^17–20 and its depolymerization.^21,22

Among depolymerization technologies, hydrogenolysis is a promising reaction to dissociate C–O interunit linkages in lignin, thereby facilitating its conversion into valuable aromatics.^23,24 While numerous studies have investigated hydrogenolysis of lignin and its model compounds, a significant limitation has been the requirement for compressed H₂ gas.^25,26 Our previous work explored the transfer hydrogenolysis of diphenyl ether (DPE), which is one of the simplest lignin model compounds.^27,28 In these studies, 2-propanol (IPA) was used as an H-donating solvent, replacing molecular hydrogen gas, and the model reaction of C–O dissociation was performed in both batch and continuous reaction systems.

For fractionation, methods designed to minimize lignin condensation and avoid harsh conditions have been developed to obtain the lignin in a suitable form for subsequent processing.^29,30 The organosolv method is one of the representatives, which typically utilizes aqueous organic solvents (e.g., alcohols and carboxylic acids) and a catalyst (base or acid) to solvolyze lignin.^31–33 Relatively milder conditions, such as less toxic reagents and lower operation temperatures, make it suitable for isolating lignin with less condensation in a more sustainable manner.^{17,31,32,34–36}

Considering the industrial-scale production of LAs from plant biomass, the integration of these fractionation and depolymerization methods is anticipated. Specifically, lignocellulose should primarily undergo fractionation, followed by depolymerization of isolated lignin. However, a significant concern involves the impact of homogeneous acids used in the organosolv process on the subsequent depolymerization step. This process commonly utilizes homogeneous acid catalysts such as HCl, which have been proven effective in facilitating lignin solvolysis and extraction, thereby enhancing process efficiency.³⁷ Conversely, these residual acids could potentially alter the behavior of the subsequent depolymerization reaction. Furthermore, these effects might be emphasized in transfer hydrogenolysis, a process highly dependent on the H-donating capability of the solvent.

Generally, both homogeneous and solid acids have been reported to contribute to the depolymerization of actual lignin either through acid-catalyzed solvolysis or by aiding in hydrogenolysis.¹¹ In the field of DPE research, the contribution of acidic sites in zeolite catalysts to transfer hydrogenolysis has been pointed out by Kumar and Srivastava.³⁸ Similarly, some studies have highlighted the contribution of acid sites on solid catalysts to the metal-catalyzed hydrogenolysis of DPE.^39,40 In contrast, the addition of homogeneous catalysts, frequently used in organosolv processes, is relatively rare in the field of hydrogenolysis of aromatic ethers (lignin models). Although Siskin et al.⁴¹ demonstrated that 92% dissociation was achieved for DPE in H₂O at 315 °C for 3 days using phosphoric acid as an acid catalyst, the reaction conditions were optimized for hydrolytic cleavage of DPE. Furthermore, studies on the hydrodeoxygenation of phenyl alcohol⁴² and a comparison of H₂ gas, IPA, and formic acid as H-donors for lignin depolymerization⁴³ have been presented; however, the influence of homogeneous acid addition on the hydrogenolysis of a simple model compound like DPE, which exhibits stability under harsh conditions and is suitable for evaluating hydrogenolytic C–O cleavage reactivity,^44,45 remains uninvestigated. Considering this background, this study aimed to elucidate the effects of homogeneous acid addition during the transfer hydrogenolysis of aromatic ethers such as DPE, benzyl phenyl ether (BPE), and phenethoxybenzene (PEB) in IPA over a Ni catalyst. The effects of HCl, formic acid (FA), and acetic acid (AA), commonly employed in organosolv processes, were tested.

2. Experimental

2.1. Chemicals and materials

Diphenyl ether (DPE, 99%), benzene (99.5%), phenol (99%), cyclohexane (98%), cyclohexanol (98%), cyclohexanone (99%), hydrochloric acid (HCl, 35–37 wt%), formic acid (FA, 98%), acetic acid (AA, 99.7%), sodium hydroxide (NaOH, 97%), n-hexane (96%), nickel formate dihydrate ((HCOO)₂Ni·2H₂O, >90%), and acetonitrile (ACN, HPLC grade) were purchased from Fujifilm Wako Pure Chemical Corporation (Japan). Benzyl phenyl ether (BPE, 98%) was purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Phenethoxybenzene (PEB, 97%) was purchased from BLDpharm (China). 2-Propanol (IPA, 99.9%) was purchased from Tokuyama Corporation (Japan). Nickel (65 wt%) on silica/alumina (commercial Ni catalyst) was purchased from Sigma-Aldrich (Japan, product number: 208779). This catalyst was proven to be effective for transfer hydrogenolysis in combination with IPA in our previous study²⁸ and was also employed in this study, considering its versatility. Fundamental properties of this catalyst have been reported in previous reports.^28,46 N₂ (99.9%) was obtained from Suzuki Shokan Co., Ltd. (Japan). CO₂ (99.5%) was obtained from Resonac Gas Products Corporation (Japan). CO (99.9%) was purchased from Air Liquide Japan. Deionized water was prepared by the Direct-Q UV 3 instrument (Merck Millipore Co., Japan). All chemicals were used as purchased/prepared without further purification.

2.2. Reaction procedure

The reaction was conducted with a hand-made batch reactor having a 10 mL internal volume. The configuration of the reactor setup was shown in our previous report.²⁷ Typically, 4.0 mL of IPA containing 7.5 mM aromatic ether (DPE, BPE, or PEB), 10 mg of Ni catalyst, a certain amount of additive, and 10 μL of n-hexane (internal standard) were loaded. The reactor atmosphere was flushed with N₂ gas 5 times, and pressurized to 0.50 MPa unless otherwise stated. The reaction was commenced by immersing the reactor in a pre-heated molten salt bath. After a set time, the reactor was transferred to a water bath to stop the reaction. The product gas was transferred to an aluminum gas bag by opening a valve of the reactor. The liquid was separated using a 0.45 μm PTFE membrane filter (Membrane Solutions).

2.3. Product analysis

The liquid samples were analyzed using a gas chromatograph with a flame ionization detector (GC-2014, Shimadzu). An InertCap AQUATIC capillary column (GL Sciences) was employed to separate the product compounds during GC analysis. Also, the solutions were analyzed using a high-performance liquid chromatograph (HPLC, JASCO) system with a UV detector (UV-2070). A DE-413 column (Shodex) was used for product separation during the analysis. The detection wavelength of the UV detector was 260 nm, and 0.50 mL min⁻¹ ACN/H₂O (68 : 32) was used as an eluent. The gas product analysis was carried out using a GC equipped with a thermal conductivity detector (GC-2014, Shimadzu). A Shincarbon-ST 50/80 packed column (Shinwa Chemical Industries Ltd., Japan) was used during this analysis. Based on the concentration of each gas product, the volume of the gas phase in the reactor, and the pressure, the moles of gas were calculated using an equation of state.

The conversion of the feedstock (X) and the product yields (Y) were evaluated based on the initial moles of the feedstock as shown in eqn (1) and (2).

3. Results and discussion

3.1. Effect of HCl addition

Fig. 1 shows the conversion and product yields after the transfer hydrogenolysis of DPE as a function of HCl concentration. In the absence of any acid additives, the reaction using IPA and a Ni catalyst accomplished 98% conversion. Concurrently, the formation of benzene (63%), cyclohexane (41%), phenol (8%), cyclohexanol (84%), and cyclohexanone (4%) was observed. The C–O bond cleavage in DPE yields benzene and phenol as primary products. Subsequently, ring hydrogenation converts benzene and phenol into cyclohexane and cyclohexanol, respectively. Cyclohexanone is also formed as a derivative of phenol. Based on these DPE degradation pathways, the reaction system demonstrated sufficient performance for the targeted transfer hydrogenolysis, although over-hydrogenation was also evident. When the same reaction was conducted in ethanol and acetone, which have less H-donation ability, no DPE conversion was observed. Similarly, conversion of DPE was not detected without the Ni catalyst. DPE has been known as a highly stable C–O model compound, which is unreactive at 315 °C for 3 days in water with a formic acid catalyst.⁴¹ Furthermore, our previous study clarified that DPE degradation co-occurs with H-donation from IPA to DPE, whereas IPA is simultaneously transformed into acetone.²⁷ Therefore, the observed DPE conversion is strongly related to the transfer hydrogenolysis reaction, and it can be used as a reactivity index. Considering these results, these conditions were selected as the standard for further evaluation of the effect of acid addition.

Fig. 1 Effect of HCl addition during transfer hydrogenolysis of DPE. Left and right columns show the yield of phenol and benzene derivatives, respectively. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM DPE in 4.0 mL IPA, and 0.50 MPa N₂. *11 mM HCl was neutralized by the same amount of NaOH. Error bars show the standard errors derived from three independent experiments.

As the HCl concentration increased, both the conversion and yields decreased. Specifically, with 11 mM HCl, the conversion of DPE was only 4%. It should be noted that the error in the conversion and yields tended to be larger in the presence of 0.55–2.2 mM HCl. This was likely because a small fluctuation in the introduced HCl concentration significantly altered the reaction performance, given that HCl is highly volatile and it is difficult to precisely control its concentration. Nevertheless, the observed trend clearly indicates that HCl makes the transfer hydrogenolysis less efficient. To investigate the underlying cause, a reaction with equimolar concentrations of HCl and NaOH (11 mM each) was conducted. Under these conditions, the conversion reached 83%, with the formation of monomeric products. HCl and NaOH are assumed to undergo neutralization and form H₂O and NaCl. Therefore, these conditions represent a neutral environment but containing ionic species. The fact that HCl inhibited the reaction, but neutralization reversed this inhibition, strongly suggests that the reaction was primarily hindered by the acidity of HCl, specifically by the presence of protons. Other ions, such as Cl⁻, appear to have a less significant impact on the reaction.

During transfer hydrogenolysis with IPA as the solvent, an equilibrium exists between IPA and acetone through a dehydrogenation reaction, which sustains the H-donating capability of the system.⁴⁷ However, if IPA is irreversibly converted into other compounds, its concentration decreases, leading to a corresponding loss of H-donation ability. For example, the dehydration of 2-propanol to propylene and di-isopropyl ether has been reported in the presence of acid catalysts.^48–50 Therefore, it was hypothesized that acidity might inhibit transfer hydrogenolysis by promoting IPA consumption. However, the conversion of IPA to non-acetone compounds was not significant in this study, with over 99% of the IPA remaining in the form of IPA and acetone. While the unintended consumption of IPA via acid-catalyzed reactions could contribute to the lower DPE conversion, it is unlikely to be the primary factor.

Another plausible explanation is that protons from the acid inhibit the reaction through competitive adsorption onto the catalyst surface. Transfer hydrogenolysis involves multiple hydrogen transfer steps. The reaction initiates with the H-donation from IPA to the catalyst surface, forming activated hydrogen species (H*), which are subsequently added to DPE, leading to its hydrogenolysis.⁵¹ The metal sites on the catalyst surface are crucial for both the generation and stabilization of H*.⁵² However, protons can also adsorb onto these metal sites, hindering the formation of H*. Furthermore, the DPE adsorption is necessary for its reaction with H*, but this adsorption can also be blocked by adsorbed protons. Considering the experimental results and the unique mechanism of transfer hydrogenolysis, which heavily relies on the H-transfer steps on the catalyst surface, the competitive adsorption of protons appears to be the main reason for the lower reaction performance.

It is noteworthy that the addition of HCl did not significantly alter the product distribution; all yields decreased proportionally with the decline in DPE conversion. However, in the presence of both HCl and NaOH, the benzene yield was notably high, while the cyclohexane yield remained minimal. This suggests that the hydrogenation of benzene was selectively inhibited. Similar to hydrogenolysis, hydrogenation also involves the H-transfer steps, requiring benzene adsorption to react with H*. Hydrogenolysis/hydrogenation of DPE and phenol has been assumed to proceed through the chemisorption of oxygen atoms onto the metal.^53,54 However, benzene is a non-polar molecule, and its interaction with metal sites is facilitated by the π-electron system.⁵⁵ Consequently, benzene was more susceptible to competitive adsorption by ions, resulting in reduced hydrogenation activity.

3.2. Effect of carboxylic acid addition

Fig. 2 presents the results of the transfer hydrogenolysis of DPE with the addition of FA and AA. Similar to HCl, the introduction of FA and AA led to a decrease in both DPE conversion and yields. In the case of FA, the conversion of DPE became 0% at 0.66 M. In contrast, the DPE conversion was as high as 7% even with 2.7 M AA. These results highlight that the intensity of the inhibitory effects varies depending on the acid type. Furthermore, as shown in Fig. 3, organic acids like FA and AA themselves underwent thermal decomposition. Particularly, it was clear that FA degraded into CO, CO₂, H₂ and H₂O, which was consistent with previous reports.^56,57 The conversion of FA reached 94% within 20 min, indicating a significant loss of acidity at this stage. However, the transfer hydrogenolysis of DPE remained inhibited, and neither higher reaction temperatures nor longer reaction times improved DPE conversion.

Fig. 2 Effect of carboxylic acid addition during transfer hydrogenolysis of DPE. (a) FA and (b) AA. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM DPE in 4.0 mL IPA, and 0.50 MPa N₂. Error bars show the standard errors.

Fig. 3 Degradation of carboxylic acids in the transfer hydrogenolysis reaction system. (a) FA and (b) AA. Conditions: 215 °C, 20 min, 0.66 M FA or AA in 4.0 mL IPA, and 0.50 MPa N₂. Note that the H₂ yield was excluded as it is also produced from IPA.

Taking the aforementioned results into account, these carboxylic acids are assumed to inhibit the reaction not only through their acidity but also through other factors. For further investigation, blank experiments using NaOH and FA salt were performed (Fig. 4). Unlike HCl, FA inhibited the DPE degradation even after neutralization by NaOH. Additionally, Ni formate, which does not possess acidity, also inhibited the reaction. These results suggest that the presence of the FA backbone in a non-acidic form hindered DPE transfer hydrogenolysis. Fig. 5 presents the effects of the head-space gases on the transfer hydrogenolysis of DPE. CO₂ and CO, the gas species generated during the degradation of the carboxylic acids, led to lower DPE conversion and yields. Notably, 1.0 MPa or higher pressure of CO almost completely suppressed the transfer hydrogenolysis. The chemisorption of CO onto transition metals is a well-known phenomenon causing catalyst poisoning.^58,59 Similar to protons from acid, adsorbed CO is likely to impede the H-transfer steps, resulting in lower conversion and yields. Based on these experimental findings, carboxylic acids, particularly FA, inhibited transfer hydrogenolysis throughout the degradation process: initially through their acidity before degradation, and subsequently through the gaseous products formed after degradation. In the case of AA, such significant gas formation was not observed, which likely accounts for the relatively higher DPE conversion in its presence due to fewer inhibitory effects.

Fig. 4 Transfer hydrogenolysis of DPE with FA, FA and NaOH, and Ni formate (FA salt). Concentrations of FA and NaOH were adjusted to 0.66 M, but 0.33 M was employed for Ni formate as it contains two (COO⁻) skeletons in one molecule. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM DPE in 4.0 mL IPA, and 0.50 MPa N₂. Error bars show the standard errors.

Fig. 5 Transfer hydrogenolysis of DPE with compressed gas. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM DPE in 4.0 mL IPA. Error bars show the standard errors.

3.3. Comparison of acid additives

To compare the impact of the different acids, a plot of DPE conversion vs. acid concentration was made (Fig. 6). The figure clearly demonstrates that the strength of the inhibition effect followed the order HCl > FA > AA. For example, 10 mM HCl inhibited the reaction, whereas DPE conversion remained relatively higher even in the presence of 1 M or more AA. As previously discussed, the primary cause of this inhibition was their acidity (proton release). Given the reported pK_a value of each acid in IPA (3.1 for HCl, 10.1 for FA and 11.4 for AA),^60–62 the observed trend is reasonable.

Fig. 6 Conversion of DPE depending on the concentration of acid additives. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM DPE in 4.0 mL IPA, and 0.50 MPa N₂. Error bars show the standard errors.

Although these pK_a values are based on measurements at room temperature and may not precisely reflect the acidity under the reaction conditions, a comparison of acids based on their relative acidity may give further insight. Subsequently, a DPE conversion vs. proton concentration plot was illustrated (Fig. 7). Interestingly, similar DPE conversions were observed at similar proton concentrations in the case of FA and AA. While the non-acidic contributions of FA and AA to reaction inhibition were evident, their acidity was also not negligible, particularly at the initial stages of the reaction. Considering the trends in Fig. 7, it is suggested that proton concentration is a contributing factor to DPE conversion efficiency. Notably, carboxylic acids showed much steeper concentration–conversion curves than HCl. This observation confirms the multiple inhibitory mechanisms of carboxylic acids: acidic and non-acidic contributions, which cannot be fully accounted by proton concentration alone. In other words, the apparent acidic contribution in Fig. 7 was overestimated, as the observed inhibitory effects included non-acidic contributions, when evaluating carboxylic acids based on proton concentration.

Fig. 7 Conversion of DPE depending on the concentration of protons from acid additives. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM DPE in 4.0 mL IPA, and 0.50 MPa N₂. Proton concentration was calculated based on the pK_a of each acid in IPA: 3.1 for HCl, 10.1 for FA, and 11.4 for AA. Error bars show the standard errors.

3.4. Comparison of aromatic ethers

DPE is commonly employed to assess the C–O bond cleavage efficiency of hydrogenolysis reactions due to its high bond dissociation energy (BDE), making it a strict model compound.⁶³ However, the proportion of the 4-O-5′ interunit motif, represented by DPE, in actual lignin is relatively low compared to other linkages such as β-O-4.⁶⁴ Considering this, analogous reactions were performed using benzyl phenyl ether (BPE) and phenethoxybenzene (PEB), which model the α-O-4 and β-O-4 linkages, respectively.

As depicted in Fig. 8, BPE and PEB were degraded to alkylbenzene derivatives and phenol derivatives, similarly to DPE. As a result, the reaction system was validated for various types of C–O linkages. Upon the addition of HCl to the reaction system, a similar effect was observed: the conversion and yield decreased significantly, indicating a comparable reaction and inhibition mechanism across these aromatic ethers. However, the conversion and yield in the presence of HCl were slightly higher for BPE and PEB compared to DPE. The BDE values for the dissociated 4-O-5 (DPE), α-O-4 (BPE), and β-O-4 (PEB) linkages are 314 kJ mol⁻¹, 218 kJ mol⁻¹, and 289 kJ mol⁻¹, respectively.⁶³ Considering these BDE values, the difference in bond strength might have become more pronounced in the presence of HCl, potentially explaining the slightly higher reactivity of BPE and PEB under acidic conditions.

Fig. 8 Transfer hydrogenolysis of aromatic ethers using IPA and a Ni catalyst with/without HCl. Left and right columns show the yield of phenol and (alkyl-)benzene derivatives, respectively. Conditions: 215 °C, 20 min, 10 mg Ni catalyst, 7.5 mM aromatic ether in 4.0 mL IPA, 11 mM HCl, and 0.50 MPa N₂. Error bars show the standard errors.

4. Conclusions

This study investigated the effects of acid addition on transfer hydrogenolysis of aromatic ethers (DPE, BPE, and PEB) in IPA over a Ni catalyst. The reaction at 215 °C for 20 min exhibited near-complete C–O scission of these aromatic ethers; however, the addition of acids such as HCl, FA, and AA resulted in a remarkable decline in C–O dissociation efficiency. The primary reason for the inhibitory effect was attributed to the protons released from acids, but the formation of gaseous products was also a contributing factor to the effects in the case of carboxylic acids. Furthermore, a comparison of acids was performed, which suggested that the inhibitory effect was generally governed by their pK_a values and proton concentration. The insights obtained in this research are important for combining the acid-catalyzed organosolv fractionation step and IPA-facilitated transfer hydrogenolysis (depolymerization) step, and thus will contribute to the development of lignocellulose refinery processes for bio-LAs production.

Author contributions

Taishi Dowaki: investigation, writing – original draft preparation, data curation, funding acquisition. Osamu Sawai: writing – review and editing, conceptualization, methodology. Teppei Nunoura: writing – review and editing, conceptualization, supervision, funding acquisition.

Conflicts of interest

The authors have no relevant financial or non-financial interests to disclose.

Data availability

All data generated and analysed in this study are included in this article.

Acknowledgements

This work was supported by JST SPRING, Grant Number JPMJSP2108, and JSPS KAKENHI, Grant Number JP25KJ0908. Also, the authors thank the Environmental Science Center and Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo for the financial support.

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