Porous silica nano-flowers stabilized PtPd bimetallic nanoparticles as heterogeneous catalyst for efficiently synthesizing guaiacol from 2-methoxycyclohexanol

Abstract

Porous silica nano-flowers (KCC-1) immobilized PtPd alloy NPs (PtPd/KCC-1) with different mass ratios of Pd and Pt were successfully prepared by a facile in situ one-step reduction, using hydrazinium hydroxide as a reducing agent. The as-synthesized silica nanospheres possess radial fibers with a distance of 15 nm, exhibiting a high specific surface area (443.56 m2·g−1). Meanwhile, the obtained PtPd alloy NPs are uniformly dispersed on the silica surface with a metallic particle size of 4–6 nm, which exist as metallic Pd and Pt on the surface of monodisperse KCC-1, showing the transfer of electrons from Pd to Pt. The as-synthesized 2.5%Pt-2.5%Pd/KCC-1 exhibited excellent catalytic activity and stability for the continuous dehydrogenation of 2-methoxycyclohexanol to prepare guaiacol. Compared with Pt or Pd single metal supported catalysts, the obtained 2.5%Pt-2.5%Pd/KCC-1 shows 97.2% conversion rate of 2-methoxycyclohexanol and 76.8% selectivity for guaiacol, which attributed to the significant synergistic effect of bimetallic PtPd alloy NPs. Furthermore, turn over frequency value of the obtained 2.5%Pt-2.5%Pd/KCC-1 NPs achieved 4.36 s−1, showing higher catalytic efficiency than other two monometallic catalysts. Reaction pathways of dehydro-aromatization of 2-methoxycyclohexanol over the obtained catalyst are proposed. Consequently, the obtained 2.5%Pt-2.5%Pd/KCC-1 NPs prove their potential in the dehydrogenation of 2-methoxycyclohexanol, while the kinetics and mechanistic study of the dehydrogenation reaction over the catalyst in a continuous fixed-bed reactor may provide valuable information for the development of green, outstanding and powerful synthetic pathway of guaiacol.

1. Introduction

Guaiacol as an important intermediate that has been extensively used in the synthesis of synthetic spices, drug and plant growth regulator, such as sulfogaiacol, vanillin [1], guaifenesin [2,3], isoprenaline [4] and 5-nitroguaiacol sodium salt. To date, various study for the synthetic pathways of guaiacol have been reported in the Refs. [5e14]. These synthesis methods could be divided into two types. One is the conventional synthesis of guaiacol in homogeneous phase and batch process, which suffers from some shortcomings like corrosive and poisonous reagents, nonreusable issues of these catalysts, complex operation, low production efficiency. Such negative issues restrict their industrial applications in a wider field. The other type is the vapor-phase selective alkylation of catechol to prefabricate guaiacol with an alkylating agent [12e17] over these heterogeneous catalytic systems, which manifest many different merits, such as high production efficiency, simple separation, environmental friendliness, etc. However, there is a common and severe problem over the tested catalysts, it tends to rapidly deactivate on the long-running experiment, greatly hindering their practical application on a large scale. Comparatively, dehydro-aromatization of 2-methoxycyclohexanol can avoid these above drawbacks over metal catalysts in a continuous fixed-bed reactor.

Therefore, more attentions have been paid to dehydrogenation of 2-methoxycyclohexanol over the supported metal catalysts because of several benefits including its excellent catalytic performance, high efficiency, simple separation procedure, and eco-friendly route. Recently, the promising strategy to the direct synthesis of guaiacol by dehydrogenation of 2-methoxycyclohexanol over the supported metal catalysts was reported in the Refs. [18e20]. Although the catalysts with only containing Pd display perfect catalytic activity in the dehydrogenation of 2-methoxycyclohexanol, their durabilities are disappointing due to carbon or coke formation. Nickel (Ni) is a cheap and widely available core metal. Nevertheless, the supported Ni monometallic catalysts show low catalytic activity, and many by-products are formed. The above-mentioned imperfections of these catalysts with containing Pd or Ni restraint their applications in the industry. Hence, to meet the growing demand of guaiacol, it is vital important and highly desirable to develop an efficient, low cost and green catalyst.