Elsevier

Renewable Energy

Volume 152, June 2020, Pages 1391-1402
Renewable Energy

Benzene, toluene and xylene (BTX) from in-situ gas phase hydrodeoxygenation of guaiacol with liquid hydrogen donor over bifunctional non-noble-metal zeolite catalysts

https://doi.org/10.1016/j.renene.2020.01.015Get rights and content

Highlights

  • Introduction of isopropanol as hydrogen donor in HDO of guaiacol in gaseous phase.

  • Catalyst Ni/HZSM-5&La demonstrates outstanding BTX and hydrocarbon selectivity.

  • The mechanism of HDO of guaiacol under hydrogen donor conditions was studied.

  • The content of hydrocarbon was over 95% after gas phase HDO of bio-oils.

Abstract

Hydrogen donors were employed to replace pure hydrogen as a hydrogen source for in-situ gas phase HDO due to their low storage and transportation cost. A series of catalysts have been prepared by modifying the zeolite HZSM-5 with polymetallic and multi-carriers. Gas-phase hydrodeoxygenation (HDO) of guaiacol was conducted under a pressure 2 MPa and a temperature at 350 °C. The effect of polymetallic and multi-carrier modification on HDO activity was investigated. The physicochemical properties of catalysts were characterized by BET, XRD, SEM, H2-TPR, XPS and NH3-TPD. The characterization results showed that when La was loaded, the catalyst Ni/HZSM-5&La obtained a stronger medium-strong acidity. The electronic transfer of the elemental nickel was promoted. After HDO, the catalyst Ni/HZSM-5&La exhibited the highest HDO rate and BTX selectivity, reaching 97.79% and 34.25%, respectively. The real bio-oil HDO under a hydrogen donor were also analyzed. The results showed the content of the hydrocarbon compound were over 95%. The reaction mechanism was discussed through the distribution of liquid product species. Direct demethoxylation pathway (DMO) and methylation pathway (ME) were considered as the main pathways.

Graphical abstract

Synopsis: A series of molecular sieve supported catalysts with different modification methods were prepared and the effect on the HDO of guaiacol was investigated under the action of hydrogen donor.

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Introduction

With the alarming rise in global energy consumption, there is an urgent need to replace fossil fuels with renewable and sustainable energy sources [1]. Therefore, the attention of the research community has centered on the abundant reserve of clean and renewable biomass [2]. As a raw source, biomass is subjected to the biomass-to-liquid process for conversion into liquid fuels known as bio-oils [3]. The setback with bio-oils, however, is that they contain high amounts of water and oxygen element, highly viscous and corrosive, and have low calorific value. Thus, they require upgrading to overcome such disadvantages and make them applicable as a fuel of existing engines. One effective and widely employed method for improving the quality and stability of bio-oils is hydrodeoxygenation (HDO) upgrading technology [4].

High-purity hydrogen has been often used as the hydrogen source for the HDO of bio-oil or various simulated compounds [5,6]. Nonetheless, the high combustion risk and the costly hydrogen production, makes hydrogen unconducive as a source for large-scale application of the HDO technology [7].

A strategy of adding an organic solvent to the bio-oil during the HDO reaction has been introduced in previous research [8,9]. Especially, a series of hydrogen-donating reactions have been investigated to determine the role of hydrogen donors (tetralin, isopropanol, glycerol, and formic acid) in lignin/microalgae, concluding that hydrogen donors have a significant effect on product distribution [10,11]. Zhang et al. studied the transfer hydrogenation of phenol over supported Pd catalysts with formic acid as the hydrogen donor. They obtained the highest phenol conversion of 65.6%, with almost all the converted phenol being turned into cyclohexanone [12]. Xiang et al. achieved 18.1%–53.1% conversion of phenol and up to 96.1% selectivity of cyclohexanone through the hydrogenation of phenol with methanol or ethanol, over Raney Ni or Pd/Al2O3 catalyst [13]. In certain conditions, the solvent can react as a co-reactant of bio-oil [14]. These studies provide another route for hydrogen donors for catalytic HDO of bio-oils.

However, a large number of studies on hydrogen donors mainly use a one-pot method, which is difficult to achieve continuous production. Studies on gas phase hydrodeoxygenation of hydrogen donors have rarely been found. And little is known about the interaction between the reactants and the hydrogen donor. The continuous HDO reactor under gas-phase conditions is more in line with industrial production requirements.

Isopropanol is selected as a hydrogen donor for HDO of guaiacol under the gas phase, and isopropanol has been proven to have excellent hydrogen supply capacity [15]. In industrial applications, the transport and safety performance of isopropanol is higher than pure hydrogen. At present, biomass pyrolysis oil is treated as dangerous waste materials. It costs $600/t to treated it and there are serious environmental problems. However, biomass pyrolysis oil is a raw material with a high C,H content, which can be used as a raw material to be converted into a chemical. The current international cost of benzene, toluene, and xylene raw materials is $615/t, $645/t and $670/t, respectively. Therefore, BTX prepared by continuous HDO of biomass pyrolysis oil under hydrogen donor conditions has high economic and environmental benefits.

Phenols are the main oxygenates in biomass pyrolysis oils and also one of the main causes of bio-oil instability [16]. Due to the presence of phenolic hydroxyl groups, phenols are most difficult to convert in the biohydrogenation process. In the biohydrogen hydrodeoxygenation process, the conversion of phenolic hydroxyl groups is critical. Therefore, guaiacol was selected as a model compound in this manuscript.

The in-situ continue HDO of real bio-oil into a BTX and hydrocarbon is promising, compared with the one-pot method. Especially, studies on gas phase hydrodeoxygenation of hydrogen donors have rarely been found, and little literature is known to the interaction between the reaction of the substrate with the hydrogen donor. The effect of dual carriers on in situ HDO of guaiacol was discussed in this manuscript. And isopropanol was selected as a hydrogen donor, and a series of catalysts were prepared to study the mechanism of guaiacol HDO. The hydrodeoxygenation reaction characteristics of real bio-oil under the condition of hydrogen donor were described, which can provide scientific basis for high-quality utilization of bio-oil. Effect of hydrogen donor isopropanol on guaiacol HDO was discussed. Furthermore, the physicochemical characteristics of the liquid-phase products of the catalyst and hydrogen donor were compared and analyzed, the role of hydrogen donors in the HDO of guaiacol was investigated, and the possible reaction mechanisms were discussed.

Section snippets

Experimental materials

Lanthanum (III) nitrate hexahydrate (AR, 99.0%), isopropanol(AR,≥99.5%), and guaiacol(GC,>99.0%) utilized in the experiments were purchased from Shanghai Macklin Biochemical Co., Ltd. Nickel (II) nitrate hexahydrate (AR, 98.0%) and copper (II) nitrate hydrate(AR) compounds were purchased from Tianjin Fuchen Chemical Reagent Factory. HZSM-5 (SiO2/Al2O3 = 54) and SBA-15 were bought from Nankai University Catalyst Co., Ltd.

Preparation of catalysts

Firstly, the zeolite carriers were calcined at 550 °C for 180 min, using a

Characterization of the catalysts

The N2 adsorption-desorption curve of the catalyst samples were shown in Fig. 1, while the specific pore structure parameters for the different catalysts were listed in Table 1. Two different types of isotherm features could be seen from Fig. 1. The specific surface area of the HZSM-5 carrier was 374.17 m2/g and had discernible type-II isotherm characteristics. Deviation from the Y-axis of the adsorption potential curve in the micropore indicated that the force against N2 was weak [22]. The

Conclusion

A series of nickel-based zeolite supported catalysts were prepared for the in-situ HDO of bio-oil. The addition of La changed the pore structure distribution on the surface of the catalyst, and improved the dispersion of the active metal Ni. The highest HDO rate(97.79%) and BTX selectivity(34.25%) were obtained via the catalyst Ni/HZSM-5&La due to the combined role of the strong acid site and better metal dispersion. When the real bio-oil was introduced, the main products form the light oil and

Authors’ contributions

En-chen Jiang and Xi-wei Xu conceived and designed the study. Pei-Dong Zhong, Ren Tu, and Yan Sun performed the experiments. Xu-dong Fan and Yu-jian Wu wrote the paper. Xi-wei Xu, and Zhi-Yu Li reviewed and edited the manuscript. All authors read and approved the manuscript.

Declaration of competing interest

There are no conflicts of interest to declare.

Acknowledgement

Supported by National Natural Science Foundation of China (Grant No.51706075); National Natural Science Foundation of China (Grant NO.51576071); Science and Technology Planning Project of Guangdong Province, China (Grant No.2016A020210073), Science and Technology Planning Project of Guangdong Province, China (Grant No.2015B020237010); Science and Technology Planning Project of Guangzhou City, China (Grant No.201906010042).

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