WO_x domain size, acid properties and mechanistic aspects of glycerol hydrogenolysis over Pt/WO_x/ZrO₂

Highlights

• The formation of 1,3-propanediol is structurally sensitive to WO_x domain size.

• Controllable preparation of active sites was achieved by suitable Mn doping.

• The intrinsic structure of active site is Pt-(WO_x)_n-H with medium sized WO_x.

• Brönsted acid on the active site is super strong, permanent and adjacent to Pt.

• Adsorption state of glycerol and active H species were confirmed by FTIR studies.

Abstract

Supported WO_x catalysts are widely investigated in glycerol hydrogenolysis for their high selectivity to 1,3-propanediol (1,3-PDO). The high performance is often related to surface Brönsted acid site. However, the intrinsic structure of Brönsted acid is unclear and its controllable preparation has not been investigated in detail. In addition, many reaction mechanisms have been proposed up to now, but with few direct evidences in spectroscopy studies. In this work, Pt/WO_x/ZrO₂ catalysts containing various amounts of WO_x were studied in glycerol hydrogenolysis. The reaction is found to be structurally sensitive to WO_x domain size, with medium polymerized WO_x shown to benefit the formation of 1,3-PDO. By doping a suitable amount of Mn into monolayer covered WO_x/ZrO₂, large amounts of WO_x with medium polymerization degree were created. Thus, the turnover frequency of 1,3-PDO (TOF_1,3-PDO/W) increased 2.6 times in comparison to the best result of none Mn-doping Pt/WO_x/ZrO₂ catalysts. Characterizations of WO_x structure and acid properties indicate that super strong Brönsted acid site is created by the interaction between medium polymerized WO_x and Pt particle. This type of acid is linearly correlated with the formation rate of 1,3-PDO. The adsorption state of glycerol was studied using infrared spectroscopy, and the secondary −OH is found to be strongly adsorbed to Brönsted acid site on WO_x containing catalyst, while its interaction with Pt/ZrO₂ is much weaker. The natural structure of the active site is proposed to be Pt-(WO_x)_n-H, integrating super strong Brönsted acid site and metallic Pt site together. Combined with FTIR investigations of different surface hydrogen species and in situ 2-propanol conversion, the reaction mechanism was also determined.

Introduction

The conversion of biomass to energy and chemicals can alleviate the pressure of fossil energy exhaustion, and also benefits the reduction of CO₂ emission because of its carbon neutral property [1]. Glycerol is one of the most important building blocks of biomass, and with the fast development of biodiesel production, its transformation becomes a hot topic in green chemistry [[2], [3], [4], [5]]. One of the routes is the hydrogenolysis of glycerol to propanediols (PDOs), which can be used as monomers in polyester industry.

Because of the high selectivity for dehydration and hydrogenation of CO structure, Cu based catalysts are widely used in glycerol hydrogenolysis to 1,2-PDO, and catalysts with 1,2-PDO yield higher than 90% have been reported by many groups [1]. However, the production of 1,3-PDO from glycerol hydrogenolysis remains a challenge. Firstly, the secondary hydroxyl group is steric hindered by two primary hydroxyls, making it less accessible to the active sites of catalysts. Secondly, for the two types hydroxyl groups, their dehydration activation energies based on neutral glycerol are similar (70.9 kcal mol⁻¹ for 2° OH and 73.2 kcal mol⁻¹ for 1° −OH), and their proton affinities are nearly the same (195.4 kcal mol⁻¹ for 2° OH and 194.8 kcal mol⁻¹ for 1° OH) [6]. Such small differences on reactivity make it very difficult to improve 1,3-PDO selectivity. In addition, because high temperatures can lead to deep hydrogenolysis of PDOs, the reaction should be carried out in mild conditions, generally below 200 °C [7]. Therefore, to achieve a better activity, strong acid especially Brönsted acid site should be the essential component of catalyst. Up to now, bifunctional catalysts are the main materials used in glycerol hydrogenolysis to 1,3-PDO, with metal sites to activate H₂ and acid sites to activate glycerol. Among them, ReO_x and WO_x based catalysts are widely studied, and the importance of Brönsted acid for 1,3-PDO formation has been emphasized for both catalysts [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17]].

Among WO_x based catalysts, WO_x/ZrO₂ is categorized as a solid super acid, which was first reported by Hino and Arata in 1988 [18]. Due to their strong acidity and great hydrothermal stability, WO_x/ZrO₂ and Pt/WO_x/ZrO₂ have been widely applied in catalytic reactions such as isomerization [19,20], NO_x reduction [21,22], alcohol dehydration [23], esterification [24,25], as well as glycerol hydrogenolysis [10,26]. A suitable WO_x loading is necessary for the good performance, and volcano curves between activity and WO_x surface density or polymerization are usually found in these reactions. Higher WO_x loading above monolayer coverage results in the formation of bulk WO₃ and decrease of the number of acid sites. So some methods have been proposed to improve the specific surface area or monolayer threshold of the materials [27,28]. The nature of the active acid site of WO_x/ZrO₂ has also been discussed for a long time. Recently, Zhou et al. overviewed previous perspectives and their comprehensive studies with the help of in situ UV–vis DRS, Raman and STEM-HAADF characterization, as well as theoretical calculations. They confirmed that three-dimensional distorted Zr-WO_x clusters (0.8–1.0 nm) are the most active catalytic species due to the strongest Brönsted acid [29]. It is worth discussing whether this model is also applicable for bifunctional Pt/WO_x/ZrO₂ catalysts in glycerol hydrogenolysis.

The reaction mechanisms of 1,3-PDO formation over Pt-WO_x catalysts are widely discussed and two routes are mainly proposed. One is dehydration-hydrogenation route, in which glycerol is firstly dehydrated to 3-hydroxy propionaldehyde (3-HPA), then the intermediate is converted to 1,3-PDO via hydrogenation [16,30,31]. The second route involves the formation of secondary carbocation intermediate (by direct dehydroxylation of 2° −OH) and follows by the attack of activated hydrogen species [12,13,15,32]. However, most mechanisms were based on product distributions over different catalysts and reaction conditions. The adsorption state of glycerol on catalyst surface and the confirmation of intermediates (3-HPA or secondary carbocation) are rarely reported. Another important issue is surface hydrogen species. It was mentioned by many groups that H₂ pressure has significant effects in glycerol hydrogenolysis [10,12,13,33], while different surface hydrogen species have also been proposed in reaction mechanisms [10,14,33,34]. Furthermore, there are various kinds of hydrogen species on the surface of Pt/WO_x/ZrO₂, and five situations are summarized here in Fig. 1: (1) Dissociative adsorbed H species on Pt metal site, (2) permanent Brönsted acid on WO_x species, (3) temporary Brönsted acid formed by the reduction of WO_x species [35,36], (4) proton and hydride pairs formed by H₂ heterolysis on WO_x species [10,12], (5) hydride captured by the ZrO₂ support [37]. Direct evidence is still needed to understand which situation contributes to the formation of 1,3-PDO.

In this work, the domain size of surface WO_x under monolayer coverage was controlled by two methods: (1) increasing the polymerization degree by changing WO_x loading, (2) decreasing the polymerization degree by Mn doping into WO_x/ZrO₂ with monolayer coverage. And its effect on 1,3-PDO formation was investigated in glycerol hydrogenolysis. The structures and acid properties were comprehensively characterized and the natural active site was proposed. FTIR studies provided evidences on the adsorption state of glycerol and surface hydrogen species that participated in the reaction. Combined with in situ study of 2-propanol conversion, the reaction mechanism was also given.