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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10165−10181
Mechanistic Approaches toward Rational Design of a Heterogeneous Catalyst for Ring-Opening and Deoxygenation of Biomass-Derived Cyclic Compounds Shelaka Gupta, Md. Imteyaz Alam, Tuhin Suvra Khan, and M. Ali Haider*
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Renewable Energy and Chemicals Lab, Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India ABSTRACT: Technologies for the processing of lignocellulosic biomass into fuels and chemicals are generally focused on selective chemical transformation of the three different types of constituents: cellulose, hemicellulose and lignin. In this regard, heterogeneous catalytic reactions are employed to defunctionalize and upgrade the platform molecules obtained selectively from these constituents. Herein, a selection of studies are discussed which are adapted to deoxygenate and valorize the biomass-derived platform molecules with a specific focus on understanding the reaction mechanisms and rational design of a heterogeneous catalyst. The selection of the deoxygenation process constituted a combination of two or three reactions. For example, ring-opening reactions of the cyclic compounds are studied with decarboxylation, dehydration, hydrogenation and/or Diels−Alder reaction carried out on metal, acid and/or oxide catalysts. The platform molecules studied here include an array of saturated lactones, 2-pyrones, cyclic ethers, furans and phenolic compounds. In the study of lactones, mechanistic insights are provided to understand the selectivity trend over the Brønsted and Lewis acid catalysts for ring-opening and decarboxylation reactions leading to the formation of α-olefins. For 2-pyrones, the integrated bio- and chemo-catalytic process is studied in which a 2-pyrone molecule obtained from fermentation media may undergo Diels−Alder, hydrogenation combined with ring-opening, decarboxylation and dehydration reactions to yield the target product. Ringopening studies on cyclic ethers, including furanic compounds, are focused on mechanistic observations in the C−O hydrogenolysis reaction, leading to the design of bimetallic alloys to produce terminal diols and carboxylic acids. In extension to this, rational thoughts on the design of bimetallic catalysts are elucidated in the hydrodeoxygenation of the phenolic compounds. KEYWORDS: Biomass, Ring-opening, Decarboxylation, Diels−Alder, Hydrodeoxygenation, Bimetallic catalysts
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boxylation reactions.21−24 In contrast to the carbohydrate part of the biomass, lignin requires a higher temperature (>400 °C) to yield constituent phenolic monomers such as guaiacol, catechol, etc.25 Nevertheless, the platform molecule obtained from the lignin part of the biomass also requires a deoxygenation step, which is primarily performed via the HDO reaction.26 This review is focused on studying such deoxygenation processes which generally include a combination of two or three reaction steps performed on heterogeneous catalysts. More specifically, mechanistic insights into the ring-opening and decarboxylation of biomass-derived cyclic esters (GVL and 2-pyrones) are provided with a focus to elaborate on concepts related to the choice and design of catalyst materials with desired functionalities. Reactions on 2-pyrones further include a discussion on the combination of DA, hydrogenation, ringopening, decarboxylation and/or dehydration reactions toward the formation of aromatic compounds. In addition, biological
INTRODUCTION Biomass as a renewable and sustainable carbon source is a promising alternative for the production of fuels and chemicals that are generally produced from fossil reserves.1−3 However, direct conversion of biomass into a desired fuel or chemical is difficult as it comprises complex molecules such as cellulose, hemicellulose and lignin.4 Aqueous phase pretreatment of biomass at mild temperatures (83%) (Table 1, entry 11).105 Furans. In order to make the overall process for the conversion of furfural to 1,5-pentanediol in one pot, a Pd catalyst may be used in combination to the bimetallic Ir-ReOx/ SiO2 catalyst, which may be helpful in hydrogenating the ringunsaturation of the furfural molecule to produce THFA. Indeed, as expected, Pd-Ir-ReOx/SiO2 catalyst was tested to give around 71.4% selectivity of the terminal diol with an absolute furfural conversion (Table 2, entry 1).107 High conversion of the furfural obtained on Pd-Ir-ReOx/SiO2 catalyst as compared to the conversion of THFA on IrReOx/SiO2 was likely due to the significant increase in reaction temperature from 100 to 200 °C (Table 1, entry 1 and 2).103,107 On adding Pd/SiO2 catalyst to the bimetallic RhReOx/SiO2 catalyst, a more direct one pot hydrogenolysis and HDO route for HMF conversion to 1,6-hexanediol was envisaged, yielding around 57.8% selectivity of the terminal diol with complete HMF conversion (Table 2, entry 2).108 In a patent filed by Boussie and co-workers, a different approach to directly convert FDCA into adipic acid was suggested.109 In
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DESIGN OF A BIMETALLIC CATALYST FOR THE HDO REACTION Bimetallic catalysts that combine the hydrogenation ability of a metal with the deoxygenation ability of the neighboring oxophilic metal were consistently observed to exhibit higher activity and selectivity for the HDO reactions.110,111 This was irrespective of any metal−support interaction where acidity of the support is desirable for the HDO reaction.99 In such bimetallic alloys, a combination of a noble and/or a non-noble metal with oxophilic metals imparted bifunctionality to the alloy catalyst. Under the hydrogenating environment, the oxophilic metal used as the modifier may be completely reduced to the metallic state or remain as an oxide. The metal oxygen bond strength for the oxophilic modifier (e.g., Co, Cr, Fe, Mo, Re, W etc.) was higher than the hydrogenating metals (Ni, Pd, Pt, Rh etc.).112 Thus, the hydrogenating site provided surface adsorbed hydrogen atoms and the oxophilic metal binded with the oxygen atom, which facilitated C−O bond scission in the adsorbed reactant. The desired effect of this bifunctionality was predominant in the HDO reactions of 10175
DOI: 10.1021/acssuschemeng.9b00734 ACS Sustainable Chem. Eng. 2019, 7, 10165−10181
Perspective
ACS Sustainable Chemistry & Engineering
Figure 9. Bimetallic alloy catalysts screened from the ab initio MKM of ethanol decomposition reaction over the step sites of the transition metal catalyst at temperature (a) 523 K and (b) 373 K. The color code represents the selectivity toward the product (ethane) obtained from the C−O bond scission of ethanol as described in reference 120.
Pd-doped Fe2O3 catalyst, which was used for conducting the HDO of m-cresol, measuring high conversion (>55%) and selectivity (>85%) toward the product toluene at 300 °C (Table 3, entry 4).116 As expected, the Fe2O3 support used was observed to reduce to the metallic Fe state in the hydrogenating environment,116 thus forming a bimetallic alloy. Here, the oxophilic metal Fe also weakened the interaction between the aromatic ring and the surface, favoring the strong binding with the carbonyl group leading to deoxygenation. An interesting case for the HDO reactions was observed in the reactivity of Zn modified Pd catalysts, for the conversion of 5HMF to DMF89 and other lignin-derived platform compounds.117 Alloying Zn with Pd, in reducing environments, significantly improved upon the yield of the HDO product.118 Prima facie Zn did not appear in the list of oxophilic metals;112 however, in the vicinity of Pd, Zn acted as the oxophilic metal with greater affinity to bind with the oxygen atom, facilitating C−O bond activation in the reactant molecule.119 The reactivity of Zn in PdZn alloy, for the HDO reaction was calculated to be more pronounced on the under-coordinated step sites as compared to the terrace surfaces.119 A more generic framework for the design of bimetallic alloy catalyst surfaces for the HDO of phenolic and other such biomass-derived platform compounds was presented by Jalid et al. 120 An ab initio microkinetic model (MKM) was constructed, specifically focusing on the reactivity of the step sites of the transition metal catalysts. In this method, ethanol was used as the model compound to study the reactivity trend of transition metal catalyst surfaces for the C−O hydrogenolysis reaction. Figure 9 represents the combination of bimetallic catalyst alloys showing high selectivity for the HDO product at a reaction temperature of 523 K. On reducing the temperature to 373 K, a different set of bimetallic alloys became more selective for the HDO product as shown in Figure 9. Thus, reaction conditions could play a critical role in the design of an active bimetallic surface for carrying out a specific HDO reaction of the biorenewable platforms.
phenolics, wherein monometallic catalysts such as Pd and Ru on an acidic support tend to hydrogenate the aromatic ring before catalyzing the HDO reaction, via an indirect HDO pathway. In contrast, the bimetallic catalysts having an oxophilic part resulted in direct HDO reaction of the phenolic and furanic compounds without saturating the ring.113 Table 3 lists such representative bimetallic catalysts which have been utilized for the HDO of biomass-derived aromatic compounds. As compared to the usual heterogeneous HDO catalysts, wherein the acidity of the support plays an important role in the HDO reaction,26 in all of the bimetallic catalysts listed in Table 3, the support had negligible role in C−O bond activation. The HDO reaction was essentially carried out by the respective functionalities of the two metals. For example, in the Pd−Fe bimetallic system used for the HDO of guaiacol, Fe was shown to act as the oxophilic metal, influencing the HDO reaction by binding strongly with the oxygen atom undergoing hydrogenolysis, resulting into the formation of benzene with high selectivity (∼83.2%).114 Using TPR, EXAFS and TEM experiments, it was convincingly demonstrated that Fe nanoparticles were completely reduced to the metallic state to form the bimetallic PdFe alloy catalyst.114 The synergistic interaction of Pd with the Fe nanoparticle played an active role in reducing Fe2+ and Fe3+ oxides, as evident in the TPR studies.114 Moreover, DFT simulations on the Fe(110) surface modified with Pd showed that the adsorption of phenol and cleavage of C−O bond was favored on the Fe sites.114 Instead of using Pd, cheaper bimetallics may be prepared by using Ni for the HDO reactions. Nie et al. had demonstrated the applicability of a NiFe bimetallic catalyst on a silica support for performing the HDO of m-cresol (Table 3, entry 2) to produce toluene.115 Interestingly, for this reaction using the NiFe catalyst, the conversion and selectivity of the HDO product were significantly reduced to 13.7% and 52.6% respectively at 300 °C as compared to the PdFe catalyst (Table 3, entry 2).115 The reasons for low conversion and product yield could be attributed to the lower reaction temperature. For the NiRe catalyst tested by Yang et al. for the same reaction and at the same process condition (T = 300 °C, P = 1 atm) the conversions were significantly higher (47.6%, Table 3, entry 3)122 as compared to the NiFe catalyst (Table 3 entry 2).115 On comparing the three bimetallics (NiFe, PdFe, NiRe), the PdFe alloy appeared to be better for the HDO of phenolics. Possibly inspired from this observation, Wang and co-workers studied the synergistic interaction of Pd and Fe in a
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CONCLUSIONS AND OUTLOOK One part of the review is focused on elucidating recently applied concepts in rationally designing a bimetallic alloy catalyst surface for carrying out HDO and ring-opening reactions of biomass-derived ethers and phenolic compounds. More specifically, cases where an oxophilic promoter was introduced as the second metal to form an alloy catalyst are 10176
DOI: 10.1021/acssuschemeng.9b00734 ACS Sustainable Chem. Eng. 2019, 7, 10165−10181
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ACS Sustainable Chemistry & Engineering Biographies
discussed. In this strategy, the oxophilic metal may introduce Brønsted acidity in an aqueous system to carry out ringopening reaction of cyclic ethers. In addition, just by having a greater affinity toward binding the oxygen atom the bimetallic catalyst may carry out the HDO reaction of the phenolic compound, giving higher product selectivity. In both cases catalyst support had negligible effect. In the other part of the review, the concept of Brønsted acidity in an aqueous environment is further expanded to the ring-opening and decarboxylation reaction of 2-pyrones and lactones generally carried out using an amorphous solid acid catalyst. Herein, different mechanistic routes are elaborated. For saturated lactones such as GVL, acid catalyst was essential for ring-opening and decarboxylation at a relatively higher temperature, T ∼ 350 °C.121,122 However, for unsaturated lactones such as 2-pyrones (TAL and 6PP) an acid catalyst was not required and ring-opening and decarboxylation was carried out at a much lower temperature (
