Introduction

Lignocellulose is a renewable resource of carbon, yielding an annual production of ~ 170 billion tons of materials, fuels, and chemicals [1, 2]. Catalytic upgradation of biomass to oxygen-containing value-added chemicals demonstrates a providential route for reducing carbon emissions and developing green chemical industries. Lignocellulose-derived furfurals (i.e., furfural and 5-hydroxymethyl furfural) can be efficiently generated from the dehydration of carbohydrates (i.e., pentoses and hexoses) [3,4,5]. These have been employed as the most critical biobased platform compounds to upgrade various fine chemicals, such as furan alcohols (i.e., furan alcohol and 2,5-bishydroxymethyl furan), furancarboxylic acids (i.e., furancarboxylic acid and 2,5-furandicarboxylic acid) [6, 7], methylfurans (i.e., 2-methylfuran and 2,5-dimethylfuran) [8, 9], linear polyols (i.e., 1,2-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,5-hexanediol, and 1,2,6-hexanetriol) [10, 11], cyclopentanones (i.e., cyclopentanone and 3-hydroxymethyl cyclopentanone), and cyclopentanols (i.e., cyclopentanol and 3-hydroxymethyl cyclopentanol) [12,13,14]. Among these, C5 cyclic compounds, including cyclopentanones and cyclopentanols, are highly desirable because of their comprehensive applications. For example, they are pivotal building blocks for various drug molecules, perfumes, polymers, and natural products with biological activities because these compounds concurrently constitute active carbonyl and hydroxyl groups in a small C5 cyclic molecule. Furthermore, they can be employed as reactants to synthesize high-density fuels through alkali-catalyzed aldol condensation reaction. Traditionally, cyclopentanone and cyclopentanol are derived through the decarboxylation–cyclization of petroleum-based 1,6-adipic acid and hydration of cyclopentene, respectively. Hydrogenative rearrangement provides an alternative synthesis route for cyclopentanones/cyclopentanols and proliferates the utilization potential of biomass.

Several research groups have reported the conversion of furfurals to cyclic compounds over metal–acid bifunctional catalysts [15,16,17,18,19,20]. Conventionally, furfural (FFA) requires performing distinct types of cascade reaction steps, such as C=O hydrogenation to furan alcohol (FA) over metal sites, rearrangement to form 4-hydroxymethyl-2-cyclopentenone (HCP) over acid sites, hydrogenation and dehydration to cyclopentanone (CPO) or further hydrogenation to cyclopentanol (CPL) (Scheme 1a). Similarly, 5-hydroxymethyl furfural (HMF) can be converted to 3-hydroxymethyl cyclopentanone (HCPN) or 3-hydroxymethyl cyclopentanol (HCPL) via the same route using the C=O-hydrogenated intermediate 2,5-bishydroxymethyl furan (BHF) and rearranged intermediate 4-hydroxy-4-hydroxymethyl-2-cyclopentanone (HHCPN) (Scheme 1b). However, some over-hydrogenated byproducts [i.e., tetrahydrofurfuryl alcohol (THFA) and 2,5-bishydroxymethyl tetrahydrofuran (THBHF)] are easily produced via furan ring hydrogenation, thus reducing the selectivity of these target cyclic compounds [21]. The efficient and selective preparation of cyclic compounds prevail a monumental challenge and inevitably requires delicate regulation of the metal and acid sites for the hydrogenation and acid-catalyzed steps. Furthermore, the cooperative effect, which plays an indispensable role in modulating catalytic activity and selectivity, requires comprehensive investigation.

Scheme 1
scheme 1

Hydrogenative rearrangement pathway of furfurals to cyclic compounds

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Reports on the synthesis of CPOs from FFAs have been well-reviewed. In particular, Dutta et al. [22] reviewed the catalytic performance of distinct types of heterogeneous catalysts and reaction conditions. However, comprehensive reviews on synergistic catalysis for promoting hydrogenative rearrangement to cyclic compounds are lacking. Herein, we review the research progress of concerted catalytic mechanisms in the hydrogenative rearrangement cascade reaction. We present the synergistic catalytic mechanism for regulating the acid support as well as for adjusting the distance between metal and acid sites and in situ hydrogen-modified active sites. Subsequently, we discuss the relation between the active sites and catalytic performance on substrate adsorption, hydrogenation activity, and acid-catalyzed activity. This review is expected to enhance the understanding of the synergetic cooperation of hydrogenation and acidic sites in advancing the progression of FFAs. Furthermore, this review can stimulate substantial deliberation in the development of comparatively efficient and/or new catalytic transformations enabled by selective hydrogenation and the acid-catalyzed synergetic mechanism, thereby providing accession for biorefineries.

Hydrogenative Rearrangement of Furfurals to C5 Cyclic Compounds

Synergistic Catalysis by Regulating Acid Support

The aforementioned reaction process indicates that the hydrogenative rearrangement of FFAs requires a metal site for accomplishing the hydrogenation step. Hronec et al. [23] introduced the green and environmentally friendly path of converting FFA to CPO. Employing water as the solvent and noble or non-noble metals supported on carbon as catalysts, they performed the reaction under a H2 pressure of 8.0 MPa at temperatures of 160–180 °C. The authors proposed the following reaction mechanism: First, FFA and H2 are absorbed and activated on the surface of the metal catalyst. Second, the C=O of FFA is hydrogenated to form the intermediate FA, and the C–OH of FA is cleaved to produce carbocation catalyzed by the H+ formed via the dissociation of water. Third, CPO is formed via the rearrangement of the carbocation and subsequent hydrogenation steps. Accompanied by the primary reaction, furanic polymers are formed on the catalyst surface via the partial polymerization of FFA (Fig. 1a). However, the reaction conditions are abrasive, and the synthesis efficiency of CPO is unsatisfactory owing to the lack of an acidic site on the carbon support. The neutral support affects the rearrangement step and causes FA accumulation, an intermolecular C–C coupling reaction that produces humins. Considering these drawbacks of the partial polymerization of FFA, reducing the reaction temperature has been the objective of many research groups who have reported various acidic metal–organic frameworks (MOFs) (such as Cu–BTC, MIL-101, MIL-100, UIO-66, and FeZn–PBA), metal oxides (ZrO2, Y2(Sn0.7Ce0.3)2O7-δ, Y2(Sn0.65Al0.35)2O7-δ/Al2O3, La2TiO7, NiMoO4, γ-Al2O3, and Ti3AlOx), layered double hydroxides (LDH), zeolites (such as Al–MCM-41, SBA-15, HY, and ZSM-5), and supported reduced metals (such as Pt, Pd, Au, Ru, Co, Ni, Cu, and Ni2P) as bifunctional catalysts for accomplishing reactions under mild reaction conditions: temperature of 120–160 °C and H2 pressure of 2.0–4.0 MPa (Table 1) [12, 13, 15, 16, 18, 23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70].

Fig. 1
figure 1

a Proposed reaction mechanism for the rearrangement of FFA to CPO; b Hydrogenative rearrangement performance of various catalysts for the conversion of HMF to HCPN; c Selective synthesis of CPO from FFA over Pd/Prussian blue. (Reproduced with permission from Ref. [18, 38]. Copyright © 2012,2019, Elsevier ScienceDirect)

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Table 1 Performance of different catalysts in the production of CPO or CPL from FFA
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Unlike FFA, regulating catalytic sites for HMF is relatively more challenging because the electron-withdrawing hydroxymethyl group of C=O-hydrogenated BHF escalates the difficulty in accomplishing the rearrangement. To illustrate, the predominant product of HMF over noble metal/MIL-101 is intermediate BHF because the weak Lewis acidity of the unsaturated coordination of Cr3+ ions is incapable of further opening the furan ring to produce HCPN [35]. Ohyama et al. [71, 72] reported that an Au/strong acidic support (such as Ta2O5 and Nb2O5) strengthens bifunctional catalysis, leading to the rearrangement of BHF and affording HCPN in 86% yield (Table 2). Designing a universal catalyst that solves the requirements for active sites is essential to drive the conversion of various FFAs. Li et al. and Deng et al. [36,37,38] have reported that noble metals supported on moderate acid MOF material catalysts (such as Fe-MIL-100, Pd/Cu–BTC, and Pd/FeZn Prussian blue (PBA)) exhibit a selective hydrogenative rearrangement for FFA and HMF at a temperature of 150 °C, a H2 pressure of 4.0 MPa, and a reaction time 6–12 h. The yield of CPO and HCPN is > 85%. However, weak Lewis acidic Pd/FeNi Prussian blue can only catalyze the synthesis of furfuryl alcohols (FA and BHF) (Fig. 1b, c). These experimental results demonstrate that acid strength plays a pivotal role in the synthesis of CPOs. To better comprehend the acidic sites on the catalyst surface, a few characterization methods were employed for further analyses (namely, NH3-temperature-programmed desorption, in situ pyridine-adsorbed FTIR, 31P magic angle spin nuclear magnetic resonance) [38, 62, 70].

Table 2 Performance of different catalysts in the production of HCPN or HCPL from HMF
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Synergistic Catalysis Through Distance Modification Between Metal and Acid Site

The precise control of nanoscale features in bifunctional catalysts has been exploited to enhance synergistic catalytic performance. FFAs have unsaturated C=C and C=O bonds; however, C=C hydrogenation is often thermodynamically favored over C=O hydrogenation. A pivotal factor for high selectivity toward CPOs/CPLs is the rational design of heterogeneous catalysts to enhance C=O hydrogenation selectivity. An efficient strategy for selective C=O hydrogenation of furan aldehydes was deployed through structural engineering of active sites such as O vacancies. Deng et al. [56] reported a series of pyrochlore-based (A2B2O7, A2B2O6O') bifunctional catalysts for the synthesis of CPOs. The pyrochlore support possesses natural O vacancies in accordance with the principle of electroneutrality. The B-site cations are easily substituted by a C-site metal cation with a lower valence, further strengthening the O vacancy concentration of the support. Bare metal ions near O vacancies provide a pure Lewis acidity to pyrochlore, and the acidity of these catalysts can be regulated by adjusting the type of metal cations [56,57,58]. In comparison to metal/traditional acid support (Pd/Hβ, Pd/TiO2, and Pd/Nb2O5), the Pd/pyrochlore exhibits evident advantages, such as acceleration of the C=O hydrogenation and rearrangement step, inhibition of furan ring hydrogenation reaction (Fig. 2a, b). Among Pd/pyrochlore catalysts (La2Sn2O7, Y2Sn2O7 and Y2(Sn0.7Ce0.3)2O7-δ), Pd/Y2(Sn0.7Ce0.3)2O7-δ exhibits the apex catalytic performance with CPO yields of > 92%. To comprehend the catalytic effect of O vacancies, in situ attenuated total reflection infrared (ATR-IR) spectra were incorporated to evaluate the molecular level interaction of FFA with the catalyst (Fig. 2c). The signal for C=O is redshifted in comparison with that for pristine FFA; however, the position of the C=O signal for the furan ring bands remains unchanged during interaction with Pd/pyrochlore, indicating that the O vacancy of catalysts can selectively absorb the O atom of C=O bond via a vertical configuration. This selective C=O adsorption guarantees selective C=O hydrogenation and inhibits the furan hydrogenation, thus reducing the byproduct yield of THFA. The adsorption kinetics data illustrate that Pd/pyrochlore possesses enhanced adsorption quality of FFA than traditional Pd/TiO2, further proving the selective adsorption of O vacancy (Fig. 2d). The reaction kinetics studies regarding the C=O hydrogenation step of FFA and the rearrangement step of FA are also performed. These studies found that the hydrogenation and rearrangement steps follow pseudo-first-order kinetics, and the concentration of oxygen vacancies strongly influences reaction rate constants (Fig. 2e, f). Oxygen vacancies not only provide the adsorption sites for C=O activation but also afford the Lewis acidic sites for the rearrangement steps. Moreover, the catalyst exhibits a stable recycling performance and generality for various FFAs, including FFA and HMF. Furthermore, Gao et al. [58] also demonstrated the universality of oxygen vacancy for tandem reactions using other pyrochlore-based catalysts (Pd/Y2(Sn0.65Al0.35)2O7-δ/Al2O3, Pd/La2TiO7, Pd/La2Zr2O7, Pd/La2Ce2O7). This study provides an effective biological preparation route for CPOs via the synergistic catalysis of metal sites and O vacancy of the support.

Fig. 2
figure 2

a Hydrogenative rearrangement performance of various catalysts for the conversion of FFA to CPO. b Synthesis of products from FFA over Pd/acidic supports and pyrochlore; c ATR-IR spectra of FFA adsorbed on Pd/Y2Sn2O7 and Pd/TiO2. d Adsorption kinetics of FFA under Y2Sn2O7 and TiO2. e Relation between FFA hydrogenation rate and number of oxygen vacancies. f Relation between FA hydrolysis rate and Lewis acid concentration. (Reproduced with permission from Ref. [56]. Copyright © 2020, American Chemical Society)

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On the surface of the aforementioned metal nanoparticles, which are supported through acidic support catalysts, the metal sites (metal nanoparticles) and acid sites (acidic support) are spatially separated, not only factoring a considerable accumulation of C=O-hydrogenated intermediates (i.e., FA, BHF) in the cascade reaction system but also limiting the cooperative catalytic performance of distinct active sites. Tong et al. [69] prepared a class of metal phosphide nanoparticles (Ni2P, CoP) for hydrogenative rearrangement reactions. Metal cations and metal atoms coexist on the nanoparticle surface. The former plays the role of a Lewis acid site, whereas the latter can be engaged as a hydrogenation site. The hydrogen activation ability and acidic performance of these catalysts can be regulated by adjusting the kind of metal cations and the topological structure (Fig. 3a, b). Interestingly, for the furfural reaction, Ni2P exhibits a 62.8% yield of CPL by the hydrogenative rearrangement route. In contrast, CoP displays a novel hydrogenative hydrolysis route with an 80.2% yield of 1,2,5-pententriol under 150 °C. According to the ATR-IR results, Ni2P nanoparticles are capable of adsorbing C=O of FFA selectively through vertical adsorption (Fig. 3c). The adsorption configuration ensures selective C=O hydrogenation over the metal sites, and the in situ-generated FA can be rapidly transferred to the adjacent metal cations for the rearrangement permitting efficient synthesis of CPL. CoP can adsorb both C=O as well as the furan ring of FFA via parallel adsorption. FFA can be converted to dihydrofuran alcohol by C=O hydrogenation and furan ring semi-hydrogenation over the hydrogenation site. The dihydrofuran alcohol can be hydrolyzed over the nearby Lewis acidic sites, preventing over-hydrogenation to THFA. The close metal site (Coδ+ and Niδ+) and acidic site (Co2+ and Ni2+) over metal phosphide provide an efficient synthesis method for CPL and demonstrate a novel synthesis route for 1,2,5-pententriol (Fig. 3d).

Fig. 3 
figure 3

a Pyridine-adsorbed FTIR spectra of metal phosphides. b MS signals of HD generation over metal phosphides during H2–D2 exchange reaction. c ATR-IR spectra of FFA adsorbed over various metal phosphides. d Proposed reaction mechanism over various metal phosphides. e FFA conversion to CPO over Ni/Al2O3 modifying with phosphorus. f H2-FTIR spectra on Sr2P2O7. g Isotope-label tracing experiment over Sr2P2O7/Ni2P. (Reproduced with permission from Ref. [60, 61, 69]. Copyright © 2021, 2023, 2021, Royal Society of Chemistry, Elsevier ScienceDirect, American Chemical Society)

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Gao et al. [60] investigated the synthesis of CPO from FFA over different nickel phosphides (Ni2P, Ni3P, and Ni12P5) supported Al2O3 (Fig. 3e). Nickel nanoparticles are common hydrogenation sites, and the introduction of phosphorus regulates the selectivity of hydrogenation products. Under optimal reaction conditions, the total yield of CPO and CPL can reach more than 90% following reaction at 150–190 °C, 3.0 MPa H2 for 2 h. As the hydrogenation site, nickel phosphides can selectively adsorb the C=O group of FFA and activate hydrogen, further catalyzing the formation of FA. The introduction of additional acidic sites further promotes the furfuryl alcohol rearrangement step and finally leads to the formation of CPO and CPL. Cheng et al. [61] constructed a Sr2P2O7/Ni2P composite catalyst for the hydrogenative rearrangement of FFAs to CPOs, proving the universality of various FFAs. The nanoscale activity of Sr2P2O7/Ni2P was crucial for the construction of the catalytic interface with modulated hydrogen activation and acid sites. They found that Sr2P2O7 can activate H2 to participate in the reaction. The H2-FTIR experiments display the formation of Sr–H and O–H bonds after H2 activation, implying that H2 could be heterolyzed on Sr2P2O2 (Fig. 3f) [76]. The mechanism was further investigated by isotope-label tracing experiments (H218O, D2O, or D2), which confirmed that both H2O and H2 participated in the transformation (Fig. 3g).

Synergistic Catalysis by Hydrogen Spillover and Heterolysis

To further enhance the intimacy of metal sites and acidic sites, Li et al. [59] and Yuan et al. [55] have constructed an intimate H+–H– pair by the hydrogen spillover mechanism. Some reducible metal oxides (such as NiMoO4 and CeO2) were used as supports to load noble metal nanoparticles. H2 can be homolyzed to hydrogen atoms on the nanoparticles, migrate to the support, and form proton H+ accompanied by the reduction of the support valence state (Fig. 4a, b). Also, the H atom on the Pd nanoparticles can be polarized into H– because of the greater electronegativity of H than Pd. The in situ-generated H+–H– pair with asymmetry can selectively adsorb and activate the C=O group of FFA to ensure the selective C=O hydrogenation and inhibit the furan hydrogenation. In addition, the generation of the H––H+ pair triggers the transformation of the original Lewis acidity to Brønsted acidity, as evidenced by in situ pyridine-adsorbed FTIR spectra (Fig. 4c). To better investigate the relation between acidity and acid catalytic ability, kinetic studies of the rearrangement of FA were conducted under H2 atmosphere using pyridine as an in situ blocking reagent for occupying both Lewis and Brønsted acid sites or 2,6-dimethylpyridine for poisoning Brønsted acid sites [77]. These results indicate that both Lewis and Brønsted acidity can catalyze the rearrangement reaction. However, the in situ-generated Brønsted acids essentially accelerated the rearrangement steps (Fig. 4d). In addition, Deng et al. [62] reported that H+–H– pairs can be generated over partial oxidized Pd/Ti3AlC2 by in situ hydrogen spillover mechanism. Similarly, the H+–H– pairs not only function as the hydrogenation sites for selective C=O hydrogenation but also provide acid sites for the acid-catalyzed step in the conversion of FFAs. As a result, the natural bifunctional properties of H+–H– pairs exhibit an 81.6% yield of CPO from FFA under 120 °C. Pd/Ti3AlC2 demonstrates excellent catalytic performance at the lowest temperature reported to date, substantially reducing the reported minimum reaction temperature for FFA conversion to about 30 °C.

Fig. 4
figure 4

a Proposed catalytic reaction mechanism. b Pd 3d, Mo 3d and O 1 s XPS spectra. c Pyridine-adsorbed FTIR spectra. d Rearrangement rate constants of various catalysts after the in situ titration by pyridine and 2,6-dimethylpyridine. (Reproduced with permission from Ref. [59]. Copyright © 2022, Elsevier Science Direct)

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Additionally, Deng et al. [70] reported a new perspective for strengthening bifunctional hydrogenation and acid-catalyzed reactions without classical acid hosts. A series of Co-embedded N-doped carbon (Co@Co–NCs) as catalysts were exploited for the conversion of FFA to CPL. The Co nanoparticle core plays a vital role in C=O hydrogenation. Intriguingly, it promotes H2 heterolysis over the Co–N pair and a subsequent water-mediated transformation of Hδ––Co–N–Hδ+ to OHδ––Co–N–Hδ+ accompanied by the generation of H2 (Fig. 5a). Namely, H2 acts as a catalyst to induce the acid–base transformation of the Co–NC shell from Lewis acid–base pairs (Co–N) into Brønsted acid − base pairs (OHδ––Co–N–Hδ+). The Brønsted acid sites of in situ-generated OHδ––Co–N–Hδ+ were confirmed by 31P magic angle spin nuclear magnetic resonance measurements, and the H+ species formed on Co–N via hydrogen heterolysis was characterized by.

Fig. 5
figure 5

a Proposed reaction mechanism over Co@Co-NC. b.13P with a probe of TMPO. c In situ IR spectra for the subsequent D2 and D2O activation at 150 °C. d MS signals of HD generation during D2–H2O treatment. (Reproduced with permission from Ref. [70]. Copyright © 2022, American Chemical Society)

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ATR-IR. Substantial HD generation is detected via mass spectrometry (MS) when Co–NC is exposed to a mixture of D2 and H2O (Fig. 5b–d). The OHδ––Co–N–Hδ+ construction selectively adsorbs and reacts with the electronegative O atom of the asymmetric C=O, promoting selective hydrogenation. The Brønsted acidity of OHδ––Co–N–Hδ+ fundamentally promotes the acid-catalyzed step. Furthermore, they demonstrated the directed synthesis of 3-methyl cyclopentanol from 5-methyl-furfural, demonstrating the remarkable universality of the selective hydrogenation rearrangement route.

In another work, multishell hollow materials were synthesized from ZIFs-MOFs with controllable nanostructures. This strategy is in accordance with the pyrolysis of multilayer solid ZIFs. The number of carbon shells and the synthesized hollow Co@Co–NC can be precisely controlled by a step-by-step crystal growth method. Due to the three-dimensional hierarchical structure, highly dispersed Co nanoparticles and nitrogen-doped carbon Co@Co–NCs can be engaged as efficient catalysts for this reaction. Multishell Co@Co–NC catalyzes the production of CPL in high yield. Under the reaction conditions of 160 °C and 2.0 MPa H2, the CPL yield reached 97.0% after 8 h reaction and demonstrated excellent cyclic stability. The optimal performance of the multishell Co@Co–NC catalyst was attributed to its structural advantages: (1) the abundant stratified holes in the hollow shell can enhance the mass diffusion capacity; (2) the high dispersion of Co nanoparticles in the hollow shell can increase the density of highly exposed active sites [65].

Summary and Perspective

This review presented a mechanistic insight into synergistic hydrogenative rearrangement of biomass-based FFAs to comprehend how the bifunctional sites (hydrogenation and acidic sites) selectively fix and activate the C=O group and cooperatively catalyze the rearrangement. (1) The medium-strength acid support–loaded metal sites are designed as bifunctional synergistic catalysts to enhance the intermediate acid-catalyzed rearrangement reaction, proving the universality of the catalytic conversion of FFAs. (2) Catalyst structure engineering of active sites (such as O vacancies) is applied for selective C=O hydrogenation of furan aldehydes and increasing the strength of acid carriers to enhance the synergistic catalysis. Reducing the distance between the hydrogenation and acid sites advances the mass transfer rate of the intermediate between these bifunctional sites. (3) In situ-generated hydrogen species via the hydrogen spillover or heterolysis mechanism results in the adsorption and activation of the O atom of the C=O group and leads to selective hydrogenation. Meanwhile, hydrogen species can also be engaged as acidic sites for the rearrangement steps. In summary, synergistic catalysis performs a vital role in the hydrogenative rearrangement reaction of biobased FFAs. This sheds light on the potential application of this strategy for the rational design of catalysts. This mechanism provides theoretical guidance for bifunctional synergistic catalysis of other highvalue-added fine chemicals derived from FFAs. Therefore, precise regulation of the reaction path of biomass resources and elaborate reaction mechanisms are contemplated to proliferate the scope for biomass refining. Future research can be focused on the following aspects.

  1. (1)

    Current reaction systems use biomass platform compounds, such as FFAs, as raw materials. Synthesis of CPO from feedstock with a higher priority (such as pentoses, rhamnose, glucose, and fructose) demonstrates better scientific and industrial prospects. This cascade of reaction strategy can avoid intermediate separation and purification and minimize energy costs by integrating several steps into a single reaction system. The reaction process would, therefore, require designing a multifunctional synergistic catalyst (with metal and acid sites) for the tandem sequence of dehydration, hydrogenation, and rearrangement steps.

  2. (2)

    Integrating biomass into the organic nitrogen-containing chemicals will reduce the carbon footprint and enhance the economic competitiveness of biorefining. The near hydrogenation and acid sites over metal phosphides can be engaged as catalysts to regulate the vertical adsorption of FA and enhance the mass transfer rate of intermediates on the catalyst surface [78]. On account of the synergic effect of efficient hydrogenation and acid catalysis, the reaction routes (FA hydrolysis, nitro hydrogenation, and the Paal–Knorr reaction) are proposed to provide an efficient method for pyrrole synthesis. Although significant progress has been made in inhibiting competitive adsorption (such as furan and nitro groups on metal sites, hydroxyl, and hydrogenated amino groups on acidic sites) over metal phosphide at tandem reaction, there are some challenges in using FFAs as reaction substrates due to strong reactivity of C=O in FFA.

  3. (3)

    Linear polyols (e.g., 1,2-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,2,6-hexanetriol) are widely used in spices, cosmetics, medical disinfectants, and cleaning industries. Thus, further research can be performed on the “oxygen-bearing strategy” to establish the method of synthesizing polyols from FFAs. Additionally, the hydrogenative ring-opening of FFAs proceeds via a “one-pot” tandem process, in which the multifunctional catalytic sites contribute to diverse reactions and ultimately lead to overall hydrogenative ring-opening transformation. It is worth noting that the high stability of these intermetallic catalysts provides the potential to solve the catalyst deactivation caused by the leaching of metal oxide in conventional metal/metal oxide catalysts.