Research paperSynthesis and characterization of 12-tungstophosphoric acid intercalated layered double hydroxides and their application as esterification catalysts for deacidification of crude oil
Graphical abstract
12-tungstophosphoric acid (HPW) intercalated LDH with the d003 value of 1.05 nm were synthesized, in which HPW may graft with the vacancies in the layer and orient in the interlayer with the C2 axis perpendicular to the layers. This phase is proved to be neither heteropolyacid salts nor polytungstate intercalated LDH as reported in most literature. Moreover, HPW intercalated LDH show better esterification activity for ethylene glycol and naphthenic acids in crude oil than their nitrate LDH precursors, due to the enhanced acidity. They may act as interlayer catalytic reactors to improve the activity.
Introduction
Polyoxometalates (POMs) are a class of molecular anionic metal-oxide clusters (Pope, 1983), which can be divided into two main categories, isopolyacids (IPAs) and heteropolyacids (HPAs). IPAs contain only transition metal ions and oxide ions, whereas HPAs incorporate one or more heteroatoms such as P5 +, Si4 + and As5 + (Pope, 1983). HPAs systems are very attractive due to the variety and selectivity of their properties and structures (Song and Barteau, 2004). One of the most important properties of HPAs is the strong acidity (Borrás-Almenar et al., 2003). In particular, the Keggin-type 12-tungstophosphoric acid (H3PW12O40, HPW), which is made up of a central PO4 tetrahedron surrounded by four trimetallic groups of three edge-sharing WO6 octahedra, is considered to be the most stable and acidic HPAs and is widely used in acid catalysis (Brahmkhatri and Patel, 2011; Baroi and Dalai, 2014).
Naphthenic acids constituted by five- or six-membered rings and one carboxyl are the dominant acids in crude oil (Barrow et al., 2009). The crude oil with total acid number (TAN) above 1.0 mg KOH·g− 1 is regarded as high acid crude oil, which leads to the serious corrosion on refinery equipment. Among various deacidification methods, catalytic esterification between alcohol and naphthenic acids has been proved to be an efficient technique, due to its low-cost, environmental pollution-free and mild reaction conditions. Development of effective esterification catalysts for deacidification of crude oil has become of great interest to researchers.
Layered double hydroxides (LDH), also known as hydrotalcite-like compounds, are a group of anionic layered materials, which can be represented by the general formula [M2 +1 − xM3 +x(OH)2]x +(An −)x/n·mH2O, where M2 + and M3 + are di- and trivalent metal cations in the host layer, respectively, An − is interlayer exchangeable guest anions (Cavani et al., 1991). Due to the acid-base properties, LDH can be used as esterification catalysts for deacidification of crude oil. Zhu's group (Huang et al., 2011; Li et al., 2013) used MgAl and ZnAl LDH as esterification catalysts to remove acids from VGO and crude oil. They found the layered structure of LDH was beneficial to the esterification activity. Our group (Wu et al., 2011; Wang et al., 2015) also found MgAl and NiAl LDH were active for esterification between naphthenic acids and ethylene glycol (EG). However, the activity of LDH catalysts needs to be further improved.
It was demonstrated that HPW intercalated LDH catalysts show greater advantages over LDH or HPW alone, for example, the acidity of LDH can be effectively enhanced (Kagunya et al., 1996; Das and Parida, 2007) and the stability of HPW can be improved (Omwoma et al., 2014). Therefore, plenty of literature exists on the preparation of HPW intercalated LDH and their application as catalysts for Knoevenagel condensation (Jia et al., 2015), dyes degradation (Jia et al., 2015), oxidative desulfurization (Yu and Wang, 2013), epoxidation of allylic alcohol (Liu et al., 2009), NO reduction (Wongkerd et al., 2008), esterification of acetic acid (Das and Parida, 2007), oxidative bromination of phenol (Parida et al., 2006), etc. However, to date, there is no report on HPW intercalated LDH as esterification catalysts for deacidification of crude oil.
In this work, HPW intercalated MgAl, ZnAl and NiAl LDH with M2 +/Al3 + molar ratio of 2 were prepared and used as esterification catalysts for deacidification of crude oil. A characteristic XRD reflection at 8.4° corresponding to a basal spacing of 1.05 nm was observed in the as-prepared HPW intercalated LDH, which is quite different to that reported in almost all literature in which such reflection was attributed to HPA salts or polytungstate intercalated LDH. Additional experiments and characterizations were performed and the results demonstrated that this reflection should be ascribed to HPW intercalated LDH. In addition, the relationship between the esterification activity and acidity of HPW intercalated LDH was studied.
Section snippets
Sample preparation
Nitrate LDH precursors (LDH-NO3) with M2 +/Al3 + (M2 + = Mg2 +, Zn2 +, Ni2 +) molar ratio of 2 were prepared by the co-precipitation method. A mixed solution containing M(NO3)2·6H2O and Al(NO3)3·9H2O (1 M in total, M2 +/Al3 + = 2) was placed in a 500 ml three-neck flask. Then, a NaOH solution (2 M) was added dropwise to the flask at 40 °C under vigorous stirring. The resulting slurry was transferred into a Teflon-lined autoclave and aged at 100 °C for 12 h. The suspension was filtered, washed several times with
XRD
The XRD patterns of LDH-NO3 and LDH-PW are shown in Fig. 1. All LDH-NO3 samples exhibit the typical patterns of layered structure with strong reflections at the lower 2θ range. The value of basal spacing of the (003) plane (d003) is 0.89 nm, in agreement with that reported in the literature (Yang et al., 2003). Compared with LDH-NO3, the (003) reflections of LDH-PW (Fig. 1b) shift to lower angles around 8.4° corresponding to the d003 value around 1.05 nm.
It's worth noting that the shortest van
Conclusion
HPW intercalated LDH (M2 +/Al3 + = 2, M2 + = Mg2 +, Zn2 +, Ni2 +) were prepared by an ion-exchange method. It is found that the LDH exhibit a characteristic reflection at 8.4° corresponding to the basal spacing of 1.05 nm. Almost all the researchers attributed such reflection to HPA salts or polytungstate intercalated LDH formed during the synthesis of HPAs intercalated LDH. However, based on the systemic experiments and characterizations, it is proved that this reflection could be ascribed to HPW
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (Project No. 21506176).
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