Preparation of tungstophosphoric acid intercalated MgAl layered double hydroxides with a tunable interlayer spacing and their catalytic esterification performance in the deacidification of crude oil
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摘要: 通过改变离子交换温度和时间合成了具有不同层间距的磷钨酸（H3PW12O40，HPW）插层MgAl水滑石（LDHs），采用XRD、FT-IR、Raman、31P MAS NMR、ICP-AES和Hammett指示剂-正丁胺滴定法等表征其性质，并研究其对模型原油的催化酯化脱酸性能。高的离子交换温度有利于形成较大的层间距（d003约1.46 nm），较长的交换时间有利于形成较小的层间距（d003约1.05 nm）。不同的层间距源自HPW在层间不同的存在形式，P2W18O626-以C2轴倾斜于层板和PW11O397-以C2轴垂直于层板的方式排列于层间时，形成d003约1.46 nm的层间距；PW12O403-与层板发生嫁接，并以C2轴垂直于层板的方向排列于层间时，形成d003约1.05 nm的层间距。层间P2W18O626-和PW11O397-能产生更高比例的中强酸中心，同时大的层间距有利于反应物扩散进入层间与酸中心接触，能够提高LDHs的催化酯化脱酸性能。Abstract: This study demonstrates the synthesis of MgAl layered double hydroxides (LDHs) intercalated with tungstophosphoric acid (H3PW12O40, HPW) by an ion exchange method, and different interlayer spacings (d003) are obtained by adjusting the ion exchange temperature and time. The crystalline structures, molecular structures, atomic compositions, acidity, and specific surface areas of the LDH samples are rigorously characterized. A relatively high ion exchange temperature is demonstrated to be favorable for the formation of a large d003 value of around 1.46 nm, while a long exchange time is favorable for the formation of a small d003 value of around 1.05 nm. The different values of d003 are the result of different orientations of HPW anions within the interlayer space. Here, d003=1.46 nm is obtained when P2W18O626- and PW11O397- anions are arranged in the interlayer with their C2 axes respectively tilted toward and perpendicular to the LDH layer planes. In contrast, d003=1.05 nm is obtained when PW12O403- anions are grafted onto the LDH layers with their C2 axis perpendicular to the layer planes. Furthermore, the catalytic esterification performance of the samples is investigated for the deacidification of a model crude oil. Compared with PW12O403- anions, the presence of P2W18O626- and PW11O397- anions in the interlayer produce a higher proportion of sites with intermediate acidity that function as catalytic sites. Moreover, a large value of d003 facilitates the diffusion of reactants into the interlayer, which enhances their contact with the catalytic sites, and thereby increases the catalytic esterification performance of the LDHs in the deacidification of crude oil.
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图 1 Mg2Al-NO3前体(a)及不同温度合成的Mg2Al-PW(b)的XRD谱图
Figure 1 XRD patterns of the Mg2Al-NO3 precursor material (a) and the Mg2Al-PWy-1 samples synthesized at different temperatures y of 25 ℃, 60 ℃, 80 ℃, and 100 ℃ for 1 h (b)
a: Mg2Al-PW25-1; b: Mg2Al-PW60-1; c: Mg2Al-PW80-1; d: Mg2Al-PW100-1
图 2 Mg2Al-NO3前体及不同温度合成的Mg2Al-PW的FT-IR谱图
Figure 2 FT-IR spectra of Mg2Al-NO3 precursor and the Mg2Al- PWy-1 samples synthesized at the different temperatures for 1 h
a: Mg2Al-NO3; b: Mg2Al-PW25-1; c: Mg2Al-PW60-1; d: Mg2Al-PW80-1; e: Mg2Al-PW100-1
图 3 不同时间合成的Mg2Al-PW的XRD谱图
Figure 3 XRD patterns of Mg2Al-PW100-z samples synthesized at a temperature of 100 ℃ for different ion exchange times z
a: Mg2Al-PW100-1; b: Mg2Al-PW100-3; c: Mg2Al-PW100-6; d: Mg2Al-PW100-12
图 4 不同Mg/Al物质的量比具有不同层间距的MgxAl-PW的XRD谱图
Figure 4 XRD patterns of MgxAl-PW100-z samples synthesized with different Mg/Al molar ratios (x = 3 and 4) at a temperature of 100 ℃ for different ion exchange times (z = 3 and 12 h)
a: Mg3Al-PW100-3; b: Mg3Al-PW100-12; c: Mg4Al-PW100-3; d: Mg4Al-PW100-12
图 5 Mg2Al-PW100-1和Mg2Al-PW100-12的Raman谱图
Figure 5 Raman spectra of the Mg2Al-PW100-1 sample (a) and the Mg2Al-PW100-12 sample (b).
图 6 Mg2Al-PW100-1和Mg2Al-PW100-12的31P MAS NMR谱图
Figure 6 31P MAS NMR spectra of the Mg2Al-PW100-1 sample (a) and the Mg2Al-PW100-12 sample (b)
图 7 不同交换时间下HPW插层MgAl LDHs的形成过程示意图
Figure 7 Schematic formation process of HPW-intercalated MgAl LDHs synthesized at a temperature of 100 ℃ with different ion exchange times z
表 1 不同交换时间和温度合成的MgxAl-PW的d003及S6/S8值
Table 1 Interlayer spacing d003 and the S6/S8 ratios of the areas under the S6 and S8 diffraction peaks for MgxAl-PWy-z samples synthesized with different Mg/Al molar ratios x, ion exchange temperatures y, and ion exchange times z
Sample Temperature t/℃ Time t/h d003 (S6)/nm d003 (S8)/nm S6/S8a Mg2Al-PW25-1 25 1 1.44 1.07 0.04 Mg2Al-PW60-1 60 1 1.44 1.07 0.05 Mg2Al-PW80-1 80 1 1.44 1.07 0.11 Mg2Al-PW100-1 100 1 1.46 1.07 0.16 Mg2Al-PW100-3 100 3 1.44 1.06 0.08 Mg2Al-PW100-6 100 6 1.44 1.06 0.04 Mg2Al-PW100-12 100 12 - 1.05 0.00 Mg3Al-PW100-3 100 3 1.47 1.10 0.13 Mg3Al-PW100-6 100 6 1.46 1.10 0.09 Mg3Al-PW100-12 100 12 - 1.10 0.00 Mg4Al-PW100-3 100 3 1.49 1.10 0.12 Mg4Al-PW100-6 100 6 1.48 1.10 0.07 Mg4Al-PW100-12 100 12 - 1.10 0.00 a: the ratio of peak area of S6 to that of S8 
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表 2 不同层间距的Mg2Al-PW的分子式
Table 2 Extrapolated chemical formulae of Mg2Al-PWy-z samples with different S6/S8 values
Sample Mg/Al(molar ratio) Formula Mg2Al-PW100-1 1.86 Mg0.65Al0.35(OH)2(PW11O39)0.02(P2W18O62)0.03(PW12O40)0.01·mH2O Mg2Al-PW100-3 1.78 Mg0.64Al0.36(OH)2(PW11O39)0.01(P2W18O62)0.02(PW12O40)0.05·mH2O Mg2Al-PW100-6 1.73 Mg0.64Al0.37(OH)2(P2W18O62)0.01(PW12O40)0.10·mH2O Mg2Al-PW100-12 1.71 Mg0.63Al0.37(OH)2(PW12O40)0.12·mH2O
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表 3 Mg2Al-NO3和Mg2Al-PW的酸性、比表面积和催化酯化脱酸性能
Table 3 Acidity, BET specific surface areas, and catalytic deacidification performances of Mg2Al-NO3 and Mg2Al-PWy-z catalysts
Sample Acid strength (H0) Amount of acidic sites /(mmol·g-1) Percentage of mid-strong acidic sites /% Specific surface area A /(m2·g-1) Deacidification ratio /% Mg2Al-NO3 0.8≤ H0 ≤7.2 0.164 - 10.3 33.9 0.8≤ H0 ≤7.2 0.601 Mg2Al-PW100-1 3.86≤ H0 ≤4.8 0.203 40.8 14.6 85.9 0.8≤ H0 ≤3.86 0.042 0.8≤ H0 ≤7.2 0.646 Mg2Al-PW100-3 3.86≤ H0 ≤4.8 0.198 36.7 17.1 82.4 0.8≤ H0 ≤3.86 0.039 0.8≤ H0 ≤7.2 0.654 Mg2Al-PW100-6 3.86≤ H0 ≤4.8 0.199 36.4 28.4 77.5 0.8≤ H0 ≤3.86 0.039 0.8≤ H0 ≤7.2 0.651 Mg2Al-PW100-12 3.86≤ H0 ≤4.8 0.197 36.1 39.2 74.2 0.8≤ H0 ≤3.86 0.038
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表 4 不同Mg/Al物质的量比MgxAl-PW的催化酯化脱酸性能
Table 4 Catalytic deacidification performances of MgxAl-PWy-z samples with different values of x
Sample Deacidification ratio /% Sample Deacidification ratio /% Sample Deacidification ratio /% Mg3Al-PW100-3 73.8 Mg3Al-PW100-6 69.1 Mg3Al-PW100-12 67.7 Mg4Al-PW100-3 71.4 Mg4Al-PW100-6 67.4 Mg4Al-PW100-12 64.5
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