Elsevier

Journal of Catalysis

Volume 380, December 2019, Pages 267-277
Journal of Catalysis

Boosting levulinic acid hydrogenation to value-added 1,4-pentanediol using microwave-assisted gold catalysis

https://doi.org/10.1016/j.jcat.2019.09.041Get rights and content

Highlights

  • Microwave assisted levulinic acid hydrogenation to 1,4-pentanediol over Au catalysts.

  • Complete selectivity to 1,4-pentanediol during solvent-free hydrogenation at 200 °C.

  • Cooperative role of Au nanoparticles and support as active sites.

  • Structural and morphological stability of the catalyst under hydrogenation conditions.

  • Au/TiO2 reused after reactivation by MW-assisted oxidation inside the reactor.

Abstract

Microwave (MW) -assisted levulinic acid (LA) hydrogenation has been performed over two gold catalysts (commercial 1 wt% Au/TiO2 by AUROlite™ and 2.5 wt% Au/ZrO2, prepared using deposition-precipitation). MW-assisted LA hydrogenation was carried out in water and in solvent-free conditions via (i) H-transfer and (ii) molecular H2. Au/TiO2 promoted complete LA conversion and the further reduction of the produced GVL to 1,4-pentanediol (1,4-PDO) in the presence of 50 bar H2 at 150 °C (4-hour reaction). Interestingly, selectivity to 1,4-PDO was complete at 200 °C. Extended characterisation highlighted the cooperative role played by the gold nanoparticles and the support, onto which activated hydrogen atoms spillover to react with LA. This results in the remarkable activity of Au/TiO2. Both catalysts showed structural and morphological stability under reaction conditions. It was possible to reactivate the Au/TiO2 catalyst by MW-assisted oxidation, paving the way for catalyst recycling directly inside the MW reactor.

Introduction

Levulinic acid (LA) is one of the most promising platform chemicals that can be obtained from biomass. It can be converted into high value-added molecules that can be used as green solvents and biofuel additives in the pharmaceutical industry and in the synthesis of biopolymers [1], [2]. Nowadays, most research studies focus on the hydrogenation of LA or levulinic esters (LEs) into γ-valerolactone (GVL), another versatile chemical, due to the preferable intramolecular esterification of LA [3], [4], [5].

The important commercial value of diols in the biodegradable-polyester and organic-synthesis industries means that several bio-derived chemicals, such as glycolide, oxalic acid, lactic acid, succinic acid and furfuryl alcohol etc., have been studied for the production of diols [6], [7], [8], [9], [10], [11].

However, there are not many reports on the further hydrogenation of GVL to 1,4-PDO. This may be due to the high stability of the lactone ring structure in GVL, which makes further hydrogenation processes difficult and requires a new catalytic system if the transformation is to be performed. 1,4-PDO is an important raw material in the chemical industry and the produced 1,4-PDO can be employed not only as a monomer for the production of polyesters, but also as a platform chemical for the synthesis of organic solvents and medicines [12]. Several bimetallic catalysts, such as Ru–Re/C [13], Rh–Mo/SiO2 [14], and Ir–MoOx/SiO2 [15], have been developed for the catalytic hydrogenation of LA or LEs. However, low selectivity to 1,4-PDO (42.3% to 82.0%) raises the costs of the process. Therefore, a non-noble metal catalyst such as Cu was prepared for the direct hydrogenation of LA to 1,4-PDO [16]. Unfortunately, the yield of 1,4-PDO was 22.0% even at a higher temperature (200 °C). The highest 1,4-PDO yield obtained from LA or LEs is 82.0% in heterogeneous catalytic systems [13]. Complete ethyl levulinate reduction to 1,4-PDO has been obtained at 160 °C using 1,4-dioxane as the solvent (6 h reaction time) with 60 bar H2 over a CuAlZn catalyst [17]. However, its toxicity and environmental impact mean that the use of 1,4-dioxane as a solvent should be avoided, according to Green Chemistry principles [18].

Although the medium can interfere with LA hydrogenation to GVL [19], the reaction generally proceeds with high yields under a H2 atmosphere in the presence of a number of solvents, such as water [20], tetrahydrofuran [21], 1,4-dioxane [19], [22], and alcohols [23], as well as under solvent-free conditions [19], [24], [25]. The reaction also takes place via catalytic transfer hydrogenation over metal oxides, and using supported metal catalysts with either formic acid [26], [27], or alcohols [28] as hydrogen donors. The reaction was reported to achieve GVL yields of 82.0–93.0% together with traces of methyltetrahydrofuran (MTHF) (<3.0%), at 250 °C, via both catalytic hydrogen-transfer hydrogenation and in the presence of 40 bar H2. In the latter case, only the combination of both sources of hydrogen guaranteed significant MTHF yields [29].

Based on these premises, the possibility of efficiently converting LA to 1,4-PDO in the presence of a catalyst that requires no solvent (solvent-free conditions) is still highly desirable, but a significant challenge.

Microwaves (MW) are currently used as a non-conventional enabling technology for the promotion of fast chemical transformations by producing rapid internal heating, which has been hypothesised to originate from the direct interaction between the electromagnetic field and specific molecules, intermediates, or even transition states in the reaction medium [30]. Indeed, there is still much debate on discerning between thermal effects, caused by the rapid heating and the high bulk-reaction temperatures that are reached under MW dielectric heating, and other specific or non-thermal effects. Such effects, which are not linked to a macroscopic change in reaction temperature, involve non-uniform heating at the surface of heterogeneous catalysts and the production of hot spots by MW irradiation, resulting in non-equilibrium local heating that is localised at the surface of the metal nanoparticles present on catalysts [31].

In this study, we will focus on MW-assisted LA hydrogenation over gold catalysts (commercial 1 wt% Au/TiO2 by AUROlite™ and 2.5 wt% Au/ZrO2 prepared by deposition–precipitation) to produce 1,4-pentanediol (1,4-PDO).

Section snippets

Materials

The commercial gold 1 wt% on titanium dioxide (AUROliteTM, catalogue number 79-0165, CAS number 7440-57-5) was provided by Strem Chemicals INC. and sold in collaboration with Project AuTEK for research purposes [32]. The gold itself is deposited on the support via a proprietary process that yields gold nanoparticles of about 2–3 nm. The catalyst is in the form of extrudates, that were crushed in a mortar until a very fine powder was obtained before the catalytic activity tests. It was compared

On the as-prepared catalysts

HR-TEM analyses were carried out on the as-prepared AuRO and AuZ catalysts, and the results are summarised in Fig. 1.

Gold nanoparticles with a spherical shape were quite homogeneously dispersed on both supports (a and c). In particular, the Au nanoparticles have a mean diameter of 2.3 ± 0.6 nm when supported on titania, whereas a mean diameter of 2.4 ± 0.7 nm was obtained for the zirconia-supported nanoparticles (Fig. 1b and 1d, respectively).

The particle-size distributions are quite narrow in

Conclusions

MW-assisted LA hydrogenation over a commercial Au/TiO2 (AuRO) catalyst and a lab-made Au/ZrO2 (AuZ) catalyst has been investigated. The reaction was performed solvent-free and in the presence of H2O. Moreover, catalytic tests were carried out by H-transfer, and in the presence of molecular H2. Several parameters, including temperature, pressure and catalyst amount, were evaluated and complete LA reduction was obtained with 100% yield to 1,4-PDO at 200 °C without any solvent with 50 bar H2 (4 h)

Declaration of Competing Interest

There are no conflicts to declare.

Acknowledgements

M. M., S.T. and G.C. are grateful for financial support from the University of Turin (Ricerca Locale 2018).

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