Gas-Phase Catalytic Transfer Hydrogenation of Methyl Levulinate with

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Gas-phase catalytic transfer hydrogenation of methyl levulinate with ethanol over ZrO2 Paola Blair Vasquez, Tommaso Tabanelli, Eleonora Monti, Stefania Albonetti, Danilo Bonincontro, Nikolaos Dimitratos, and Fabrizio Cavani ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06744 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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ACS Sustainable Chemistry & Engineering

Gas-phase catalytic transfer hydrogenation of methyl levulinate with ethanol over ZrO2 Paola Blair Vásquez, Tommaso Tabanelli*, Eleonora Monti, Stefania Albonetti, Danilo Bonincontro, Nikolaos Dimitratos, Fabrizio Cavani* Dipartimento di Chimica Industriale ‘‘Toso Montanari’’, Università di Bologna, Via Risorgimento 4, 40136 Bologna, Italy H-transfer, alkyl levulinates, zirconia, bio-ethanol, gas-phase, continuous flow. * [email protected], [email protected]

ABSTRACT

This manuscript reports about the gas-phase reduction of methyl levulinate to -valerolactone via catalytic transfer hydrogenation using ethanol as the H-donor. In particular, high-surface-area, tetragonal zirconia has proven to be a suitable catalyst for the reaction. Under optimised conditions, the reaction is selective toward the formation of GVL (yield 70%). However, both the deposition of heavy oligomeric compounds over the catalytic surface and the progressive conversion from Lewis to Brønsted acidity, due to the reaction with the water formed in-situ, led to a progressive change in the chemo-selectivity, promoting side reactions, e.g. the alcoholysis of angelica lactones to ethyl levulinate. However, the in-situ regeneration of the catalyst performed by feeding air at 400°C for 2 h permitted an almost total recovery of the initial catalytic behaviour,

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proving that the deactivation is reversible. The reaction has been tested also using a true bioethanol, derived from agricultural waste. Introduction Over the last several decades, the scientific community has been faced with a switch from petrochemical feedstock to renewable, abundant, and economic alternatives. From this perspective, lignocellulosic biomass is a valuable raw material for the production of bio-based chemicals at an affordable price.1,2 Lignocellulosic biomass is a potential source for a vast array of bio-derived platform molecules in this context;3,4 amongst them, levulinic acid (LA) is of particular interest. In fact, its two functional groups make LA a versatile intermediate for the synthesis of various organic (bulk)-chemicals; for this reason the United States Department of energy has classified LA as one of the top 12 most promising bio-based building block chemicals.5 LA can be derived from lignocellulosic biomass via a multi-step approach. First, cellulose is depolymerised by hydrolysis to yield glucose in the presence of an acid solution. Glucose can then undergo isomerisation to yield fructose, and – after dehydration (to hydroxymethylfurfural, HMF) and subsequent rehydration under the same acidic conditions – it yields LA with formic acid as the co-product (see Scheme 1).6,7 OH O HO OH

OH

HO O

OH Pretreatment

O

Acid hydrolysis

O OH

Acid catalyst

OH

n

Isomerisation

O

HO HO

HO HO

OH OH

HO

OH

Fructose

Glucose

Cellulose

O

Dehydration Acid catalyst

O OH O

O

O

Levulinic acid

+

H

Hydration

OH

Formic acid

Acid catalyst

HO

O

HMF

Scheme1. Levulinic acid reaction pathway starting from cellulose.


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LA upgrading for the production of valuable chemicals and biofuels has been widely investigated using different synthetic strategies; the main routes are esterification, oxidation, and reduction reactions, as shown in Scheme 2. O

O O tetrahydrofuran

O

O 5-nonanone

valerolactone

O

O 5-methyltetrahydrofuran

SOLVENTS

O O angelicalactone

O ethyl levulinate

FUELS CHEMICAL INTERMEDIATES

FOOD, FLAVOURING AND FRAGANCE COMPONENETS

O HO Succinic acid

OH

O

O

HO

PLASTICISERS HO OH

acrylic acid

O OH

OH

1,4-pentanediol

O sodium levulinate

OH

HO diphenolic acid

Na

R

O

OH

ANTI-FREEZE AGENTS O O

OH

RESINS

O Levulinic acid

1,4-butanediol

R

POLYMERS PHARMACEUTICAL AGENTS O HO

O

Br O

5-bromolevulinic acid

O

HERBICIDES

O

O OH

HO

H N

N H

Nylon 6,6 (polyamide)

O aminolevulinic acid

Scheme 2. Levulinic acid valorisation to value-added derivatives. This way, LA can be used as a source of polymer resins (e.g. diphenolic acid, acrylic acid, Nylon 6,6), as components of flavours and fragrances (e.g. ethyl levulinate and α-angelica lactone), textile dyes, solvents (e.g. tetrahydrofuran and γ-valerolactone), and additives for fuels (e.g. 5-nonanone, 5-methyltetrahydrofuran), antimicrobial agents, herbicides (e.g. δ-aminolevulinic acid), and plasticisers (e.g. 1,4-butanediol, 1,4-pentanediol).7 Of particular interest is the conversion of LA/levulinate esters through cyclisation/hydrogenation into γ-valerolactone (GVL). GVL is a 5-carbon cyclic ester that is considered an important


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building-block chemical for organic synthesis, a promising solvent, and also an additive for perfumes, foods, and fuels.7,8 The catalytic hydrogenation of LA/levulinate esters is a key step in the production of GVL,9 either via hydrogenation to yield 4-hydroxyvaleric acid followed by cyclisation to give GVL, or via acid-catalysed dehydration of LA to α-angelica lactone and consequent hydrogenation to GVL.10 Unlike LA, levulinate esters have lower boiling points and acid-free characteristics, which make them an easier alternative starting source for producing GVL. Moreover, the acid-catalysed alcoholysis of carbohydrates has been shown to give higher yields for levulinate esters.11,12 Hence, the catalytic conversion of levulinate esters appears more attractive from an industrial standpoint.13 Recently there have been several different reports on the catalytic transformation of LA and levulinate esters to GVL. Some research groups have reported the use of homogeneous catalysts10; however, this does not seem to be the most suitable approach for the production of GVL due the complexity involved in the separation and recovery of the catalyst and purification of the products.14 Therefore, an alternative and more suitable approach for this transformation is offered by the development of heterogeneous catalysts. The most common approach has been catalytic hydrogenation using molecular hydrogen in the presence of noble metal nanoparticles deposited on high surface area supports, which permits a complete conversion and GVL selectivity of 9899%.15 The use of noble catalysts such as Ru16,17 makes it possible to reach satisfactory yields in GVL (e.g. 92%), but an important disadvantage is the high cost of the catalyst. In addition, the studies published rely on fossil-carbon-based hydrogen as the reductant source,18 which makes it less sustainable.19 For these reasons, a different approach has been recently investigated, namely catalytic transfer hydrogenation (CTH) using the Meerwein−Ponndorf−Verley (MPV) procedure. CTH uses organic molecules, especially alcohols, that may function as hydrogen donors in the


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presence of a proper catalyst with Lewis acid/base pair properties.13,20,21,22 In particular, secondary alcohols, such as isopropanol, are more suitable for this procedure because of the greater stability of the carbocation formed during the reaction. Once again, noble metal catalysts have been studied for the transformation of LA to GVL. For example, Kuwahara et al.23 studied the CTH of methyl levulinate (ML) over a Ru(OH)x/TiO2 catalyst in the liquid phase with isopropanol as the hydrogen donor, reaching an almost total conversion of ML and a GVL yield of 80%. Along similar lines, Yang et al.24 tested carbon-supported Ru nanoparticles for the CTH of ethyl levulinate (EL) using isopropanol as a H-donor, reaching a GVL yield of 93%. They were able to further improve the yield of GVL to 99% by changing the catalyst and using Raney Ni in a batch reactor and working at very mild reaction conditions (rt to 80°C), but using a great excess of isopropanol. According to Komanoya et al.25, an efficient MPV reduction can take place over ZrO2 due to both the Lewis acid strength and the high density of basic sites on the surface. This way, a synergistic effect of the two sites on the simultaneous activation of both the carbonyl group and the alcohol is achieved. ZrO2 is a suitable, economic alternative to the noble metal catalyst reported on so far. Hence, several studies have focused on the use of ZrO2- and ZrO2-supported metals for the CTH of levulinate esters. He et al.13 evaluated the catalytic performance of ZrO2-supported aluminium for the CTH of EL using different alcohols as H-donors: namely methanol, ethanol, 1- and 2propanols, and cyclohexanol. Isopropanol gave the best catalytic results, achieving an EL conversion of 95.5% and a GVL yield of 83.2%. Zr(OH)4 was also tested as a potential catalyst for the CTH of EL with different alcohols. Once again, isopropanol gave the best results: EL conversion of 93.6% and a GVL selectivity of 94.5%.26 However, secondary alcohols may lead to the formation of the corresponding ketones that may produce unwanted side reactions. Hence, C1C2 alcohols such as methanol and ethanol have attracted scientific research.

21,27,28

Based on


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mechanistic studies, it has been reported that a primary alcohol is less prone to undergoing hydride shift29, because the carbocation formed is highly unstable. On the other hand, ethanol is an attractive H-donor because of its abundance, sustainability, non-toxicity, and environmentally benign nature, and it has rarely been applied to CTH.24,30,31 Nevertheless, supercritical ethanol has been proven to work also as a hydrogen donor in the presence of ZrO2, reaching an EL conversion of 95.5% and a GVL yield of 81.5%. This result, however, was achieved using harsh reaction conditions (250°C and 70 bar of autogenic pressure).31 Overall, most studies published for the CTH of LA and its esters and ZrO2 catalyst have been performed in a liquid phase using batch reactors. In the past few years, however, many successful examples of CTH reduction of bio-based building blocks in gas-phase, continuously fed fixed-bed reactor have been proposed.28,32–34 These systems make it possible to work in a wide range of reaction temperatures at atmospheric pressure, thus increasing the reactivity and productivity. In this study, we report on the previously unexplored gas-phase CTH of alkyl levulinates (methyl and ethyl levulinate, ML and EL, respectively) over ZrO2, using bio-ethanol as the hydrogen donor, the latter being both chemical-grade ethanol a real mixture derived from agricultural waste (namely from molasses and cereals fermentation) provided by Caviro, a leading Italian wine producer. Reaction conditions were optimised and long-term stability and catalyst deactivation were investigated.

Experimental section Materials Reagents and standards were analytical grade, in particular: acetonitrile ≥99.9%, octane 98%, methyl levulinate ≥98%, γ-valerolactone 99%, α-angelica lactone 98%, ethanol ≥99.8%,


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zirconium (IV) oxynitrate hydrate 99%, ammonium hydroxide solution 28-30% (NH3 basis), methyl pentanoate 99%, and ethyl levulinate 99%, all obtained from Sigma Aldrich and used as received. Commercial, monoclinic ZrO2 was purchased from Sigma Aldrich (CAS: 1314-23-4; Aldrich code: 204994). Biomass-derived ethanol was provided by Caviro, a leading wine Italian producer group. This bio-ethanol was a real mixture derived from agricultural waste (namely from molasses and cereals fermentation) with the following rough volumetric composition: ethanol 95%, acetic acid 1,3%, ethyl acetate 1.2%, methanol 1.8%, aldehydes and acetals 0.7%.

Catalyst Preparation Tetragonal ZrO2 was prepared by the precipitation method reported by Chuah et al.35 The following experimental procedure was followed: a solution of ZrO(NO3)2∙2H2O (Sigma Aldrich) 0.3 M was added dropwise to a stirred solution of NH4OH 5M at room temperature. The solution was then digested at 100°C for 24 h under a reflux system. The pH of the solution was adjusted to 9 during digestion by the dropwise addition of NH4OH 28 %w/w. The precipitate was then separated by filtration and washed with NH4OH 5 M. Lastly, the filtered sample was dried at 100°C overnight and then calcined in static air at 500°C for 12 h with a heating rate of 2.5°C/min. Catalyst Characterisation BET, specific surface area: The specific surface area of the catalysts was determined by N2 absorption–desorption at liquid N2 temperature using a Sorpty 1750 Fison instrument. 0.25 g of the sample was typically used for the measurement, and the sample was outgassed at 150°C before N2 absorption.


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X-ray diffraction analyses (XRD): XRD powder patterns of the catalysts were recorded with Nifiltered Cu Kα radiation (λ = 1.54178 Å) on a Philips X'Pert vertical diffractometer equipped with a pulse height analyser and a secondary curved graphite-crystal monochromator, in the range of 5°

Gas-Phase Catalytic Transfer Hydrogenation of Methyl Levulinate with

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