Hydrodeoxygenation of guaiacol over Pt-Ga-mesoporous catalysts

https://doi.org/10.1016/j.micromeso.2020.110815Get rights and content

Highlights

  • Pt-Ga-SBA-15 is very active in guaiacol HDO.

  • Gallium incorporation as heteroatom into the siliceous support improved Pt nanoparticles dispersion.

  • Guaiacol conversion increases with temperature, but HDO ratio reaches a maximum at 200 °C and then decreases.

  • Acidity modifies the path of reaction, favoring the HDO route through the aromatic-OCH3 and aromatic-OH bond cleavage.

  • The main product of the reaction was phenol followed by anisole and catechol.

Abstract

The hydrodeoxygenation (HDO) of guaiacol as model compound has been studied in a batch reactor over SBA-15 and Ga-SBA-15 supported platinum catalysts at 12 atm of H2 and different temperatures. Gallium incorporation as heteroatom into the mesoporous siliceous framework modified the nature of the support generating Brønsted acidity. The catalysts were characterized by means of X-ray diffraction, N2 isotherms, XPS, TPR, CO chemisorptions, Py FTIR and TEM. Good structure, narrow pore size distribution and high platinum dispersion were achieved in the synthesized catalysts. The catalysts were very active in the conversion of guaiacol at 200 °C, almost complete conversion was obtained in 3 h of reaction. Mild acidity generated by gallium incorporation enhanced the dispersion of platinum active species and improve the HDO ratio.

Introduction

Pyrolysis is an old thermochemical transformation that in recent years has captivated scientific community because this is a viable way to convert agricultural and forestry waste in liquid fuels becoming a profitable process. Lignocellulose is a structural component present in plants and this way it is the most abundant, inexpensive and renewable biomass all over the world [1]. It has three main components: cellulose, hemicellulose and lignin. Bio-oils obtained from lignocellulose by fast pyrolysis often have high oxygen contents (O/C≈) and they are characterized by low heating values, poor stability, high viscosity and corrosiveness [2,3]. From a chemical point of view lignin-derived compounds are too complex to be used as liquid fuel and require upgrading, either with catalytic hydroprocessing [[3], [4], [5], [6], [7]], zeolite upgrading [3,5,8] or aqueous phase processing [6,8]. Hydrodeoxygenation (HDO) has been proposed as a promising process to enhance the liquid product quality. HDO occurs in presence of a catalyst upgrading bio-oils to hydrocarbon via oxygen removal. HDO studies usually employ model compounds such as phenol, anisole and guaiacol to simulate bio-oil, considering those molecules the most representative because they are in large proportion in bio-oils derived from lignocellulosic biomass. Guaiacol (2-methoxyphenol) has propensity for coke formation, it is the most difficult to deoxygenate [9] and has two representative oxygenated functional groups (methoxy and phenolic). Many researchers have applied traditional hydrotreating (HDT) catalysts (sulfided Co–Mo/Al2O3 or Ni–Mo/Al2O3) for the HDO of guaiacol [3,10]. However, these industrial hydrotreating catalysts have some disadvantages for HDO of guaiacol because sulfur contamination of products may occur and consequently the catalyst deactivation by surface sulfur stripping [11,12]. Thereby, non-sulfided catalysts are receiving more attention, transition-metal-based catalysts such as Ni/ZrO2 [13], Ni/SiO2 [14,15], MoN/Al2O3 and MoN/SBA-15 [13], Fe/SiO2 and Fe/AC [16], Sn/CNF/Inconel [17], V2O5/Al2O3 [18], Ni/W/TiO2 [19] and noble metal catalysts (Pt, Pd, Rh and Ru) supported on activated carbon, alumina and siliceous mesoporous materials [[20], [21], [22], [23], [24], [25], [26], [27]].

HDO activity, as well as, product selectivity is influenced by the nature of the support [28,29]. Ghampson et al. [23] reported that there are clear differences between alumina and SBA-15- supported catalysts in terms of products distribution. Mo nitride over alumina-supported catalysts produces mostly catechol, while the SBA-15-supported catalysts produced more phenol. The acidic properties of alumina may be responsible for catechol formation. The active phase is responsible for de catalytic activity; however, the support modified the nature of the active sites of nitrides and affected the product selectivity. Mesoporous silica materials, such as SBA-15, are interesting as supports in heterogeneous catalysis due to their highly ordered and uniform sized-nanopores, large specific surface areas (above 1000 m2 g−1), thick framework walls and high thermal stability. In addition it is possible to control the morphology, pore size and composition depending on the preferred application. Pure SBA-15 has a neutral and regular surface leading to weak metal-support interactions. The incorporation of a metallic heteroatom into the siliceous mesoporous matrix may generate alterations over the support surface. Our previous results [[30], [31], [32]] indicated that Ga confers weak acidity to the SBA-15 surface in agreement with literature [33]. Mild acidity in the support could improve the activity of the catalyst in HDO of guaiacol because of the interaction between the support and the active species, eluding coke formation. According to literature platinum has high activity and stability for hydrogenation of aromatics at low reaction temperatures and moderate hydrogen pressures [34].

In this work we study the activity of Pt-Ga-SBA-15 catalysts on the HDO of guaiacol, we also evaluate the influence of the support modification with gallium as heteroatom and compare its activity with Pt-SBA-15.

Section snippets

Synthesis of the catalysts

The synthesis of mesoporous SBA-15 followed the procedure described by Zhao et al. [35]. Gallium incorporation with Si/Ga = 10 was performed according to our previous work [30]. The incorporation of platinum nanoparticles over SBA-15 and Ga-SBA-15 was made by wet impregnation method. Platinum precursor used was chloroplatinic acid (H2PtCl6xH2O). A solution of chloroplatinic acid in ethanol was prepared at 50 °C under reflux in order to obtain a nominal content of 0.5 wt% of Pt in the final

Structural and textural properties

Structural characterization results of the samples are shown in Fig. 1. Low angle XRD and type IV-H1 [36] N2 adsorption-desorption isotherms demonstrated SBA-15 structure in all the synthesized samples. Wide-angle XRD (inset Fig. 1a), showed a characteristic signal of metallic Pt (JCPDS-702075) at 39.8° in Pt-SBA-15 corresponding to the phase (111) [37,38]. High gallium dispersion is suggested since no Ga2O3 peaks were observed (JCPDS card: 43–1012 of Ga2O3). The same occurs in the case of Pt

Conclusions

We have prepared SBA-15 and Ga-SBA-15 supported platinum catalysts and analyzed their activity in HDO of guaiacol at different temperatures. Gallium incorporation as heteroatom into the siliceous support improved Pt nanoparticles dispersion, because Ga generates weak acidity over the surface of SBA-15 conferring to the catalyst new acid properties. The results showed that complete guaiacol conversion is reached in a short reaction time at 12 atm and 200 °C using Pt-Ga-SBA-15 with 0.5% wt. of

CRediT authorship contribution statement

Lorena Rivoira: Data curation, Writing - original draft. María L. Martínez: Methodology, Data curation. Andrea Beltramone: Conceptualization, Methodology, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

NANOTEC, CONICET, Universidad Tecnológica Nacional, Maestro López y Cruz Roja Argentina. We acknowledge the financial support provided by CONICET Argentina, PIP CONICET 11220120100218CO.

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