Deactivation behavior on VPO and VPO-Zr catalysts in the aldol condensation of methyl acetate and formaldehyde
Graphical abstract
Introduction
Methyl acrylate (MA) and acrylic acid (AA), which are important industrial monomers, are extensively used to produce plastics, leather treatment agents, adhesives, coatings, PAN fibers, and nylon-66 [1]. MA and AA are synthesized by two-step oxidation of propylene. In this process [2,3], propylene is oxidized to form acrolein, which is then converted into AA. Scholars have attempted to fabricate MA through vapor-phase aldol condensation [[4], [5], [6], [7], [8], [9], [10], [11]] between formaldehyde (FA) and methyl acetate (MAc). Vanadium phosphorous oxide (VPO) catalysts exhibit excellent performance in selective oxidation of n-butane, isobutene, propane, or ethane to corresponding acids, and also perform well in the aldol condensation of FA and MAc to form MA [[12], [13], [14], [15], [16], [17], [18]].
In the recent years, VPO catalysts prepared by organic solvents showed improved catalytic performance in n-butane oxidation reaction [19,20]. Feng et al. [20] reported VPO catalysts prepared by employing organic solvent exhibiting higher conversion of MAc to MA in aldol condensation. In our study, under same reaction condition, higher MA yield have been further obtained by adding Zr4+ ion as dopant (VPO-Zr). It has been shown that the suited active phases and IV4+/IV5+ ratio are the main reasons for the improvement of catalytic behavior [21]. However, less research related deactivation and reactivation behaviors was done for a multiple cyclic used of VPO catalyst.
In general, excellent performances catalyst tends to induce faster deactivation, which has two main deactivation routes for deactivation behavior [[22], [23], [24]]. One route is the removal of metallic oxides which was flowed away with mobile phase or sintered after long time of reaction, and the deactivation is irreversible. The second route of deactivation is caused by carbonaceous species which is adsorbed on active sites and blocked pore, then result in catalytic activity decrease. These deactivation species of catalyst is reversible by burning off the residual carbon species. Carbon deposition is the most common reason for catalysts deactivation. The difference of coke location and species results in a different impact on regeneration activity of the used catalysts. For same reaction system, the differences of carbon deposition distribution and species were suspected to be caused by the diverging pore system. Due to the different diffusing restrains in the microporous system and mesoporous system, causing accumulation of coke on the outer sphere of the particles or on the bottom of pores, respectively [25].
In this study, the deactivation/reactivation behavior and the certain correlation of carbon formation and time in the aldol condensation of FA with MAc to form MA on VPO and VPO-Zr catalysts were investigated. This work aims to obtain necessary information regarding deactivation/reactivation behavior and its relation to catalyst activity. The fresh and deactivated catalysts were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen physisorption, and thermogravimetric analyses. The reasons of catalytic deactivation were investigated. The empirical Voorhies formula was used to fitting the relation between coke content and reaction time.
Section snippets
Materials
1,3,5-trioxane (≥99.0%), PEG-6000 (≥99.0%), Benzyl alcohols (≥99.5%) and phosphoric acid (≥85%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Methyl acetate (≥99.5%) and iso-butyl alcohol (≥99.9%) was purchased from Tianjin chemical reagent factory. Vanadium pentoxide (≥99.7%), zirconium nitrate (99.9%) were provided by Handing Co., Ltd. All chemical agents were used as received with no further purification.
Preparation of fresh catalysts
The VPO catalyst was prepared by introducing organic solvent [26]. Briefly, a
1 catalytic lifetime
The aldol condensation of FA and MAc to MA was used as the reaction to study the deactivation and reactivation behavior of the VPO and VPO-Zr catalysts in three reaction–regeneration cycles [27]. Unless noted otherwise, all these catalysts (3 g,20–40 mesh) were run at 643 K with a liquid hourly space velocity (LHSV) of 1.0 h−1 (the molar ratio of MAc and FA is 5). In these cycles, reversible coke was considered to be totally eliminated by regenerating the catalyst through coke combustion with
Conclusion
The deactivation/reactivation behavior of VPO and VPO-Zr catalysts in the aldol condensation of MAc and FA to MA was investigated. The time-dependent deactivation on the various chemicals for VPO and VPO-Zr catalysts were detected. The empirical Voorhies formula was used to fitting the relation between coke content with reaction time. And the correlations of carbon formation and reaction temperature were also obtained on the catalysts. The used catalysts were analyzed by a variety of
Acknowledgments
The authors gratefully acknowledge financial support from the National Science Fund for Excellent Young Scholars (No. 21422607) and the National Natural Science Foundation of China (No. 21676270), NSFC-Key Projects of Shanxi Coal Based Low Carbon Joint Fundation (No. U1610222), the National Key Projects for Fundamental Research and Development of China (2016YFB0601303), the CAS/SAFEA International Partnership Program for Creative Research Teams, and the Key Laboratory of Applied Surface and
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