Effects of calcination temperature on the catalytic performance of Ti(SO4)2/CS for DME direct oxidation to polyoxymethylene dimethyl ethers
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摘要: 采用等体积浸渍法制备了活性炭球(CS)负载Ti(SO4)2的双功能催化剂，考察了焙烧温度对Ti(SO4)2/CS催化剂选择氧化二甲醚(DME)直接合成聚甲氧基二甲醚(DMMx)催化性能的影响。结果表明，不同焙烧温度制备的Ti(SO4)2/CS催化剂表现出明显的催化活性差异，280 ℃焙烧的30% Ti(SO4)2/CS催化剂具有最佳性能，DME转化率为11.7%，DMM1−3的选择性达到75.8%，其中，DMM2, 3选择性在30%以上。采用SEM、XRD、Raman、TG、NH3-TPD及XPS等表征手段研究了催化剂的结构及表面性质。焙烧温度改变了活性炭球表面官能团的分布，进而影响了Ti(SO4)2的分散状态，调变了酸中心种类和数量，尤其是弱酸和中强酸的比例，使催化剂表面酸性强弱呈现不同梯度，催化剂的酸性和氧化还原性位达到比较合理的匹配，进一步促进了C−O链的增长。。
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关键词:
- 二甲醚 /
- 直接氧化 /
- 聚甲氧基二甲醚 /
- 焙烧温度 /
- Ti(SO4)2/CS
Abstract: A series of Ti(SO4)2/activated carbon spheres (CS) bifunctional catalysts were designed and prepared by impregnation method, and the effect of calcination temperature of the catalysts on direct oxidation of dimethyl ether (DME) to polyoxymethylene dimethyl ethers (DMMx) was investigated. The results showed that the performance of Ti(SO4)2/CS catalysts was closely related to the calcination temperature. The 30% Ti(SO4)2/CS catalyst calcined under O2 atmosphere at 280 ℃ exhibited excellent activity over which the conversion of DME reached 11.7% with the selectivity of DMM1−3 up to 75.8%, wherein, the selectivity of DMM2−3 was over 30%. The texture and surface properties of the catalysts were characterized by SEM, XRD, Raman, TG, NH3-TPD and XPS. The suitable amount of weak acid sites and redox sites of the Ti(SO4)2/CS were beneficial to the direct oxidation of DME to DMMx. The calcination temperature changed the distribution of functional groups on the surface of CS which then affected the dispersion form of Ti(SO4)2. The type and amount of acid centers especially the ratio of weak acid and medium strong acid could also be adjusted, which can lead to different gradients of the surface acidity of the catalyst. The reasonable matching of the acidic and redox sites on the catalyst can evidently promote the growth of C−O chain. -
图 1 不同放大倍数下CS和30% Ti(SO4)2/CS SEM照片(O2，280 ℃)的SEM照片
Figure 1 SEM images of CS and 30% Ti(SO4)2/CS (O2, 280 ℃) with different magnification
图 2 不同焙烧温度制备的Ti(SO4)2/CS催化剂的XRD谱图和Raman谱图
Figure 2 XRD (a) and Raman (b) patterns of Ti(SO4)2/CS catalysts with different calcination temperature
图 3 活性炭球和30% Ti(SO4)2/CS(O2, 280 ℃)催化剂在空气气氛下的热重曲线
Figure 3 TG curves of CS and 30% Ti(SO4)2/CS (O2, 280 ℃) in air atmosphere
图 4 不同焙烧温度制备的Ti(SO4)2/CS催化剂的NH3-TPD谱图
Figure 4 NH3-TPD spectra of Ti(SO4)2/CS catalysts with different calcination temperature
图 5 Ti(SO4)2/CS催化剂的XPS全谱谱图
Figure 5 XPS survey spectra of Ti(SO4)2/CS catalysts with different calcination temperature
图 6 不同焙烧温度制备的Ti(SO4)2/CS催化剂的XPS-C 1s谱图
Figure 6 XPS-C 1s spectra of Ti(SO4)2/CS catalysts with different calcination temperature
表 1 焙烧温度对Ti(SO4)2/CS催化性能的影响
Table 1 Effects of calcination temperature on the performance of Ti(SO4)2/CS for DME oxidation to DMMx
Catalyst DME Conv. x/% Selectivity sC-mol/% DMM DMM2 DMM3 DMM1−3 CH3OH HCHO MF COx CS 11.4 trace 0.0 0.0 trace 100 0.0 0.0 0.0 30% Ti(SO4)2/CS(O2, 240 ℃) 11.4 72.8 1.8 0.0 74.6 14.7 0.0 1.7 9.0 30% Ti(SO4)2/CS(O2, 280 ℃) 11.7 43.3 30.4 2.1 75.8 20.7 0.3 0.6 2.6 30% Ti(SO4)2/CS(O2, 320 ℃) 11.7 60.4 14.3 0.0 74.7 20.6 0.0 0.8 3.9 30% Ti(SO4)2/CS(O2, 360 ℃) 10.6 80.0 0.0 0.0 80.0 16.5 0.0 2.5 1.0 reaction conditions: 240 ℃, atmospheric pressure, cat: 1 mL, 3600 h−1, nDME:nO2 = 1:1, DMM: CH3OCH2OCH3;
DMM2: CH3OCH2OCH2OCH3; DMM3: CH3OCH2OCH2OCH2OCH3; MF: HCOOCH3
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表 2 NH3-TPD量化
Table 2 Quantitative analysis of NH3-TPD measurements
Catalyst AwA AMSA AMSA/AwA CS 108 1059 9.81 30% Ti(SO4)2/CS(O2, 240 ℃) 618 21555 34.88 30% Ti(SO4)2/CS(O2, 280 ℃) 14913 14586 0.98 30% Ti(SO4)2/CS(O2, 320 ℃) 28077 9736 0.35 30% Ti(SO4)2/CS(O2, 360 ℃) 20944 2494 0.12 AwA: area of weak acid; AMSA: area of medium strong acid
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表 3 不同温度焙烧的Ti(SO4)2/CS催化剂的XPS C 1s谱图分析
Table 3 XPS C 1s spectra analysis of Ti(SO4)2/CS catalysts with different calcination temperature
Catalyst C−C/C−H w/% C−O−C/C−OH w/% C=O w/% COOH w/% π→π* w/% CS 63.84 20.51 3.82 5.00 6.85 30% Ti(SO4)2/CS(O2, 240 ℃) 57.45 26.86 4.56 6.13 5.00 30% Ti(SO4)2/CS(O2, 280 ℃) 63.22 17.66 9.09 4.23 5.81 30% Ti(SO4)2/CS(O2, 320 ℃) 56.25 19.70 8.44 4.25 11.36 30% Ti(SO4)2/CS(O2, 360 ℃) 58.70 21.01 5.00 5.92 9.36
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