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ZHU GuangYou, WANG Meng. Novel Approach for Molecular Characterization of Trace Metalloporphyrins in Crude Oils[J]. Acta Sedimentologica Sinica, 2023, 41(2): 610-618. doi: 10.14027/j.issn.1000-0550.2021.079
Citation: ZHU GuangYou, WANG Meng. Novel Approach for Molecular Characterization of Trace Metalloporphyrins in Crude Oils[J]. Acta Sedimentologica Sinica, 2023, 41(2): 610-618. doi: 10.14027/j.issn.1000-0550.2021.079
shu

Novel Approach for Molecular Characterization of Trace Metalloporphyrins in Crude Oils

doi: 10.14027/j.issn.1000-0550.2021.079
  • 1.

    Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China

Funds:

China National Petroleum Corporation Forward-looking Basic Strategic Technology Project 2021DJ0504

  • Received Date: 2021-03-11
  • Accepted Date: 2022-03-11
  • Rev Recd Date: 2022-01-29
    • Available Online: 2022-03-11
  • Publish Date: 2023-04-10
  • Metalloporphyrins in crude oil are of great significance since they reveal the organic/inorganic interactions and oceanic anoxic events during paleoenvironmental change. To date, few studies have been reported on the molecular composition of metalloporphyrins in high-maturity crude oils with metal content < 10 mg/kg. In this study, a new matrix with ionization energy 1.3 eV above that of metalloporphyrins was synthesized for the matrix-assisted laser/desorption ionization （MALDI） source to promote ionization of the metalloporphyrins. Metalloporphyrins in high-maturity crude oil （equivalent Ro=1.79 to 2.42） from the Tarim Basin were enriched by liquid⁃liquid extraction. The molecular composition of the trace metalloporphyrins in these crude oils with metal content <4.0 mg/kg was analyzed by MALDI combined with Fourier Transform ion cyclotronic resonance mass spectrometry （FT-ICR MS）. A total of 10 O2 vanadium porphyrins, seven O3 vanadium porphyrins and one O1S1 vanadium porphyrin were newly discovered and identified in the Tarim oils. Simultaneity analysis of the trace vanadium porphyrins oxidized and nickel porphyrins confirmed the presence of metalloporphyrins in highly mature crude oils. The formation mechanism of the new metalloporphyrins is also discussed.
  • [1] Speight J G. The chemistry and technology of petroleum[M]. 5th ed. Boca Raton： CRC Press, 2014： 123-125.
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  • Received:  2021-03-11
  • Revised:  2022-01-29
  • Accepted:  2022-03-11
  • Published:  2023-04-10

Novel Approach for Molecular Characterization of Trace Metalloporphyrins in Crude Oils

doi: 10.14027/j.issn.1000-0550.2021.079
  • 1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
Funds:

China National Petroleum Corporation Forward-looking Basic Strategic Technology Project 2021DJ0504

  • Received Date: 2021-03-11
  • Accepted Date: 2022-03-11
  • Rev Recd Date: 2022-01-29
  • Available Online: 2022-03-11
  • Publish Date: 2023-04-10

Abstract: Metalloporphyrins in crude oil are of great significance since they reveal the organic/inorganic interactions and oceanic anoxic events during paleoenvironmental change. To date, few studies have been reported on the molecular composition of metalloporphyrins in high-maturity crude oils with metal content < 10 mg/kg. In this study, a new matrix with ionization energy 1.3 eV above that of metalloporphyrins was synthesized for the matrix-assisted laser/desorption ionization （MALDI） source to promote ionization of the metalloporphyrins. Metalloporphyrins in high-maturity crude oil （equivalent Ro=1.79 to 2.42） from the Tarim Basin were enriched by liquid⁃liquid extraction. The molecular composition of the trace metalloporphyrins in these crude oils with metal content <4.0 mg/kg was analyzed by MALDI combined with Fourier Transform ion cyclotronic resonance mass spectrometry （FT-ICR MS）. A total of 10 O2 vanadium porphyrins, seven O3 vanadium porphyrins and one O1S1 vanadium porphyrin were newly discovered and identified in the Tarim oils. Simultaneity analysis of the trace vanadium porphyrins oxidized and nickel porphyrins confirmed the presence of metalloporphyrins in highly mature crude oils. The formation mechanism of the new metalloporphyrins is also discussed.

ZHU GuangYou, WANG Meng. Novel Approach for Molecular Characterization of Trace Metalloporphyrins in Crude Oils[J]. Acta Sedimentologica Sinica, 2023, 41(2): 610-618. doi: 10.14027/j.issn.1000-0550.2021.079
Citation: ZHU GuangYou, WANG Meng. Novel Approach for Molecular Characterization of Trace Metalloporphyrins in Crude Oils[J]. Acta Sedimentologica Sinica, 2023, 41(2): 610-618. doi: 10.14027/j.issn.1000-0550.2021.079
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0.   引言

  • 原油是一种复杂的有机集合体,不仅包括碳氢化合物,还包括含硫,含氮,含氧化合物以及多达45种金属化合物[1]。金属卟啉是一类广泛分布在原油、沥青和油页岩中的生物标志化合物[2]。Treibs最早在页岩和原油发现了钒（VO,IV）卟啉和镍（Ni,II）卟啉[3⁃4],自此研究人员开始对这类化合物进行了较为广泛研究。其中初卟啉（etioporphyrins,简称ETIO）和脱氧叶红初卟啉（deoxophylloerythroetiop⁃orphyrins,简称DPEP）是原油中结构最简单,含量最丰富的两种金属卟啉化合物（图1）。ETIO和DPEP的等效双键数（double bonds equivalent,简称DBE）分别为17和和18。DBE是用于衡量化合物不饱和度的参数,可利用式（1）计算。

    DBE=c+1-h2-x2+n2 (1)

    式中：c,h,x,n分别为分子式中碳原子,氢原子,卤素原子,氮原子的数量。研究人员还利用紫外—可见（UV-Vis）光谱和质谱（MS）表征了低成熟度地层和重质原油中二环脱氧叶红初卟啉（dicyclic-deoxophyllo erythroetioporphyrins,di-DPEP）,苯并初卟啉（rhodo-etioporphyrins,rhodo-ETIO）,苯并脱氧叶红初卟啉（rhodo-deoxophylloerythroetioporphyrins,rhodo-DPEP）和苯并二环脱氧叶红初卟啉（rhodo-dicyclicdeoxophy lloerythroetioporphyrins,rhodo-di-DPEP）[5⁃7]。金属卟啉化合物作为一类稳定的生物标志物,记录了原油的来源与演化信息。除了DPEP与ETIO卟啉的相对含量用于衡量原油成熟度以外,地质样品中卟啉的分子组成信息,对于进一步揭示古环境变化过程中有机—无机相互作用及海洋缺氧事件具有重要意义[8]。

    Figure 1.  Molecular structural formula of etioporphyrin and deoxophylloerythroetioporphyrin common in crude oil

    紫外—可见光谱是一种常用的卟啉鉴定和定量分析方法,该方法对金属卟啉具有较高敏感性。但是由于原油是一种极其复杂的混合物,很难表征低浓度的金属卟啉。因此,原油样品需要进行分离和浓缩预处理,以减少样品中其他化合物的干扰。研究人员开发了多种表征金属卟啉的方法,包括溶解度分级分离与索氏抽提法[9],真空升华法[10⁃11],以及包括高压液相色谱（HPLC）和薄层色谱（TLC）在内的色谱法[12⁃15]。但是,这些方法的共同不足之处是流程繁琐,耗时较长,重复性差。

    近年来,傅里叶变换离子回旋共振质谱（FT-ICR MS）被逐步用于分析复杂原油的分子组成。FT-ICR MS具有很高的分辨率和分析精确度,能够确定质谱中每个峰的元素组成和分子式[16⁃19]。Rodgers et al.[20]利用正离子电喷雾电离源（+ ESI）FT-ICR MS针对智利塞罗内格罗超重原油中镍卟啉和矾卟啉进行分子组成分析,McKenna et al.[7]利用大气压光电离源（APPI）FT-ICR MS对加拿大阿萨巴斯卡油砂沥青中的钒卟啉进行了鉴定。Qian et al.[21⁃22]通过APPI FT-ICR MS对某减压渣油的沥青组分（钒金属含量为2 020 mg/kg）和加利福尼亚陆上石油的沥青组分（镍金属含量497 mg/kg,钒金属含量734 mg/kg）中的镍卟啉和钒卟啉进行了鉴定。Zhao et al.[23⁃24]利用索氏抽提法和固相色谱法富集了委内瑞拉重质原油（20 ℃下密度为1.03 g/cm3,钒金属含量513 mg/kg）中的钒卟啉,随后利用+ ESI FT-ICR MS鉴定了多达11种新型钒卟啉化合物。上述这些研究均认为,即便使用超高分辨率质谱对金属含量高达200×10-6的重油及其沥青组分中的卟啉化合物进行分析,也需要事先花费大量时间和流程对金属卟啉进行分离富集。此外,Bonnett et al.[25⁃26]还在澳大利亚维多利亚州、美国蒙大拿州、巴基斯坦Makerwal地区以及土耳其Canakkale-Can地区的褐煤中分离并发现了铁卟啉。另外,Woltering et al.[8]利用制备型HPLC在澳大利亚Eromanga盆地的早白垩世的低成熟II型烃源岩露头和Bight盆地的侏罗纪—白垩纪海相黑色页岩中分离富集了金属卟啉,随后用离子阱质谱（Orbitrap MS）同时鉴定了镍卟啉、钒卟啉、铜卟啉、锌卟啉和锰卟啉等5种金属卟啉。Zheng et al.[27]利用索氏抽提从得克萨斯州Maverick盆地的鹰滩组低成熟海相黑色页岩中分离富集了富含金属卟啉的沥青组分（铁金属含量634.2 mg/kg,钒金属含量270.1 mg/kg,镍金属含量257.7 mg/kg,镓金属含量29.3 mg/kg）,然后通过+ ESI FT-ICR MS同时鉴定了钒卟啉、镍卟啉、铁卟啉和镓卟啉等4种金属卟啉。综上所述,目前鉴定的绝大部分金属卟啉化合物集中于重油（及其沥青组分）和低成熟的泥页岩中,在分析前大多需要经过一种甚至数种分离方法将样品中的金属含量富集至200×10-6以上,才能够利用先进的高分辨质谱技术揭示地质样品中金属卟啉化合物的分子层次信息。目前,在金属卟啉化合物分析领域困扰研究人员主要有4个难题：1）在高成熟原油中有无金属卟啉化合物；2）在轻质原油（密度小于1.0 g/cm3）中有无金属卟啉化合物；3）怎样在金属含量小于10 mg/kg地质样品实现金属卟啉的分子组成分析；4）如何有效提高分离富集金属卟啉的效率。

    基于上述4个难题,针对基质辅助激光解吸电离源（matrix assisted laser desorption ionization, MALDI）FT-ICR MS研发了新型电子转移基质α-CNPV-CH3,大幅提高了金属卟啉的电离效率,简化了金属卟啉的富集流程,并成功应用于塔里木轻质高成熟原油中痕量金属卟啉化合物的分子组成分析。

1.   材料与方法

    1.1.   试剂与基质的合成

  • 乙腈、二甲基甲酰胺（DMF）、三乙胺（TEA）、双二亚苄基丙酮钯（Pd（dba）2）、亚磷酸三苯酯（P（OPh）3）、1-溴-4-甲基苯、2,2’-（1,4-亚苯基）二丙烯腈苯、乙酸乙酯、八乙基镍卟啉、四苯基钒卟啉等试剂均购自Sigma-Aldrich（纯度均大于99.9%）。如图2所示,通过Mizoroki-Heck偶联反应[28]合成α-CNPV-CH3（图2c）,在N2气氛下向干燥DMF中,分别加入0.1 mmol的1-溴-4-甲基苯（图2a）,0.05 mol的2,2’-（1,4-亚苯基）二丙烯腈（图2b）,0.05 mol的Pd（dba）2,1mol的亚磷酸三苯酯和0.5 mol的三乙胺,所得混合物在120°C下搅拌反应2 h。将反应产物倒入50 mL浓度为5%的盐酸中,得到淡黄色沉淀物。收集沉淀物后在乙酸乙酯中重结晶得到α-CNPV-CH3（图2c）纯品。利用Gaussian软件（2009版）对α-CNPV-CH3、八乙基镍卟啉、四苯基钒卟啉的电离能进行计算,具体细节参考文献[29⁃30]。

    Figure 2.  Synthesis route of the matrix α⁃CNPV⁃CH3

    • 1.2.   原油的金属含量测定

  • 原油的金属含量是通过电感耦合等离子体质谱（ICP-MS,Thermo公司的XSERIES型）进行测定的,称量1.0 mg原油,利用硝酸进行硝解处理后得到的产物然后溶于水,配制成水溶液按照文献中方法进行金属含量测定[27]。

    • 1.3.   MALDI

  • FT⁃ICR MS分析

    将1.0 mL原油与2.0 mL乙腈混合后,在50 ℃下充分震荡30 min,静置分层后取乙腈相并浓缩至约100 μL。取50 μL浓缩液加入10 μL四氢呋喃和10 μL已稀释至浓度为5.0 mmol/L的α-CNPV-CH3的乙腈溶液,混合均匀后滴在MALDI-FT-ICR-MS的钢靶上待分析。MALDI FT-ICR MS是Bruker公司的Solarix型仪器,磁场强度15.0 T,分辨率为8 M（m/z 400）,激光采用Nd:YAG （355 nm）,光斑为中等,能量设为35%。采集m/z（质核比）为150~1 000范围内的信号,每张分析的扫描次数为300。谱图校准及归属采用MALDI-FT-ICR-MS配置的DataAnalysis软件,金属卟啉DBE与碳原子数关系图采用OriginPro软件处理。

2.   基质与卟啉的电离能

  • MALDI的电离机理是电子转移[31⁃32]。该机理分为两步（图3）,首先基质（M）吸收光电子（hv）生成阳离子自由基（M+）,然后进行待分析分子（A）反应生成阳离子自由基（A+）。经过理论计算,本文合成的基质α-CNPV-CH3的电离能约为8.42 eV,八乙基镍卟啉和四苯基钒卟啉的电离能分别6.50 eV和7.03 eV。McCarley et al.[31]的理论计算、实验结果和机理解析（图3）表明,MALDI有效电离待测分子的条件是基质电离能比待测分子高出0.5 eV。基质α-CNPV-CH3较2种常见金属卟啉高出1.39 eV,为高效电离痕量金属卟啉奠定了良好的基础。

    Figure 3.  Ionization mechanism of MALDI

3.   典型原油中痕量金属卟啉分析

  • 塔里木原油TZ12-4的金属钒和镍的含量分别为3.30 mg/kg和2.37 mg/kg,原油密度为0.906 g/cm3,等效镜质体反射率（Easy %Ro）为2.27,属于高成熟度原油[33]。本文利用开发的基质α-CNPV-CH3和MALDI FT-ICR MS,结合液液萃取方法对TZ12-4原油中的金属卟啉分子组成分析。TZ12-4原油的m/z=200~600范围内的FT-ICR质谱图及选取的m/z=557.15~557.26范围内放大图显示,在m/z=557.15~557.26范围内检测到3个钒卟啉化合物（图4）,分别是DBE=24的C34H26N4VO,DBE=18的C32H34N4VO2和DBE=17的C33H38N4VO2（图中分子式为分子离子形式）。3个化合物质谱峰的信噪比（S/N）为4.1~9.7,检测误差（error）为-0.260~-0.449,均小于±1 mg/kg,说明利用MALDI FT-ICR MS确定化合物分子质量具有极高的精确度和分辨率。根据得到的分子式,图中还给出了推测的分子结构式,从左到右分别为钒二苯并脱氧叶红初卟啉、氧化钒卟啉和钒初卟啉。

    Figure 4.  MALDI FT⁃ICR MS broadband mass spectrum at m/z = 200 to 600 for TZ12⁃4 crude oil (insert shows enlarged selected mass spectrum at m/z = 557.15 to 557.26)

    在TZ12-4原油中检测到的40个钒卟啉化合物和2个镍卟啉化合物（表1）。钒卟啉化合物包括DBE=17~25的含有1个氧原子的钒卟啉（简写为O1钒卟啉）,DBE=17~21的含有2个氧原子的钒卟啉（简写为O2钒卟啉）,DBE=18~21的含有3个氧原子钒卟啉（简写为O3钒卟啉）,DBE=25的含有1个氧原子和1个硫原子的钒卟啉（简写为O1S1钒卟啉）和DBE=19~20的镍卟啉。

    序号分子式卟啉种类DBE实测质量数理论质量数质量误差/（mg/kg）信噪比
    1C25H22N4VOO1钒卟啉17445.122 43445.122 77-0.7644.1
    2C32H36N4VO17543.232 16543.232 32-0.29521.9
    3C33H38N4VO17557.247 79557.247 97-0.3239.7
    4C35H42N4VO17585.279 29585.279 270.0346.7
    5C36H44N4VO17599.294 69599.294 92-0.3846.3
    6C47H66N4VO17753.467 15753.467 070.1064.4
    7C27H24N4VO18471.138 19471.138 42-0.48811.8
    8C28H26N4VO18485.153 86485.154 07-0.43311.4
    9C29H28N4VO18499.169 6499.169 72-0.24013.8
    10C30H30N4VO18513.185 17513.185 37-0.39013.9
    11C31H32N4VO18527.200 86527.201 02-0.3039.1
    12C32H34N4VO18541.216 67541.216 670.0004.6
    13C27H22N4VO19469.122 62469.122 77-0.3204.7
    14C27H20N4VO20467.107 33467.107 120.4504.6
    15C30H26N4VO20509.153 90509.154 07-0.3346.9
    16C31H28N4VO20523.169 61523.169 72-0.2109.8
    17C33H32N4VO20551.200 52551.201 02-0.9074.9
    18C38H36N4VO23615.232 16615.232 32-0.2605.5
    19C34H26N4VO24557.153 82557.154 07-0.4494.1
    20C36H30N4VO24585.185 47585.185 370.1714.1
    21C41H40N4VO24655.264 20655.263 620.8855.2
    22C35H26N4VO25569.154 45569.154 070.6684
    23C27H26N4VO2O2钒卟啉17489.148 84489.148 985-0.2965.7
    24C28H28N4VO217503.164 51503.164 635-0.24810.3
    25C29H30N4VO217517.180 18517.180 285-0.2037.4
    26C30H32N4VO217531.195 77531.195 935-0.3116.8
    27C28H26N4VO218501.148 99501.148 9850.0104.4
    28C29H28N4VO218515.164 49515.164 635-0.2817.7
    29C30H30N4VO218529.180 06529.180 285-0.4259.1
    30C32H34N4VO218557.211 44557.211 585-0.2606.9
    31C29H24N4VO220511.133 79511.133 3350.8905.4
    32C28H20N4VO221495.101 60495.102 035-0.8794.7
    33C30H30N4VO3O3钒卟啉18545.175 05545.175 199-0.2734.3
    34C34H38N4VO218585.242 60585.242 885-0.4874.1
    35C40H50N4VO318685.332 27685.331 6990.8334.6
    36C39H46N4VO319669.299 76669.300 399-0.9554.2
    37C41H50N4VO319697.332 15697.331 6990.6474.4
    38C33H32N4VO320583.191 21583.190 8490.6194.9
    39C38H40N4VO321651.253 22651.253 449-0.3524.8
    40C46H48N4VOSO1S1钒卟啉25755.298 12755.298 291-0.2265.9
    41C30H28N4Ni镍卟啉19502.166 13502.166 191-0.1214.5
    42C26H18N4Ni20444.088 27444.087 9410.7414.1

    图5展示了在TZ12-4原油中检测到的4类钒卟啉和镍卟啉的碳原子数与DBE的关系图,图中实心圆点的面积大小代表了化合物离子信号强弱,面积最大圆点的无因次相对强度为40,面积最小圆点的无因次相对强度为1。各自选取一个典型化合物推测其分子式（图5a~e）,其他化合物的结构式未全部列举。可明显看出,钒卟啉以DBE=17（ETIO）和DBE=18（DPEP）的小类占优,O2钒卟啉同样以DBE=17和DBE=18的小类占优,而O3钒卟啉,O1S1钒卟啉和镍卟啉无明显含量占优势的小类。在TZ12-4原油中检测到的4类钒卟啉和镍卟啉,均有碳原子数不连续分布现象,这是由于样品中金属卟啉丰度低、检测信号弱导致的。

    Figure 5.  Relative ion abundance plots (DBE vs. carbon number) of (a) O1 vanadium porphyrins; (b) O2 vanadium porphyrins; (c) O3 vanadium porphyrins; (d) O1S1 vanadium porphyrins; and (e) nickel porphyrins, as well as the speculative molecular structures of the selected porphyrins in each type

4.   塔里木原油中痕量金属卟啉分析

  • 将该方法用于塔里木盆地其他10个高成熟原油（密度为0.856~0.970 g/cm3,等效Ro为1.79~2.42）痕量金属卟啉的分子组成分析。虽然这些原油的金属钒仅为0.35~2.60 mg/kg,金属镍含量仅为0.26~1.65 mg/kg,但新方法成功实现了金属卟啉的分子组成分析。每个原油中检测出的金属卟啉的种类及其数量列于表2。与TZ12-4原油类似,19个原油中O1钒卟啉种类最多,其次为O2钒卟啉,O1S1钒卟啉和O3钒卟啉较少；钒卟啉的数量普遍多于镍卟啉,这与原油中金属镍含量整体低于金属钒含量有关,也与镍卟啉比钒卟啉更难电离有关[21,23]。该结果证实高成熟度原油仍然存在金属卟啉生物标志物。

    编号原油样品密度/（g/cm3）等效Ro/%金属钒含量/ mg/kg金属镍含量/mg/kg金属卟啉种类
    O1钒卟啉O2钒卟啉O3钒卟啉O1S1钒卟啉镍卟啉种类合计
    1TZ14-20.9062.273.302.37221161242
    2LN14-KH0.9621.991.911.1214401524
    3LN570.9651.791.490.9311133422
    4RP30130.9252.311.350.7810313219
    5XK4-30.9362.152.011.066314418
    6XK8-10.9422.192.601.6511120418
    7FY1010.9232.251.020.664412516
    8TZ4-6-100.8562.421.671.526404216
    9Ha6c0.9701.870.350.268112315
    10HD25-10.9471.970.860.7710001314
    11LN2-34-50.9661.850.870.574402111
5.   塔里木原油中痕量金属卟啉分析

  • 如图6所示,笔者推测上述新发现的钒卟啉和镍卟啉,均是在早期沉积、成岩与成烃过程中,无机矿物中的镍钒离子对输入有机质中的原始生物标志物（如叶绿素,血红素）进行地球化学修饰的结果[34]。叶绿素、血红素等原始生物标志物的结构在成烃过程中,发生失去植醇、脱羧、芳构化、金属交换和烷基转移等物理化学反应,并最终生成了O2钒卟啉、O3钒卟啉、O1S1钒卟啉及镍卟啉[34⁃35]。这些金属卟啉化合物代表的其他地球化学与地质意义还在进一步研究。

    Figure 6.  Speculative geochemical formation process of oxidized O2 vanadium porphyrins, O3 vanadium porphyrins, O1S1 vanadium porphyrins and Ni porphyrins

6.   结论

  • 针对MALDI FT-ICR MS开发了一种新型电离基质α-CNPV-CH3,电离能高达8.42 eV,能够有效促进原油金属卟啉的电离。使用简单的液液萃取方法富集了塔里木盆地的11个原油中的金属卟啉。基于MALDI FT-ICR MS和α-CNPV-CH3实现了塔里木盆地原油（金属钒含量小于4.0 mg/kg,金属镍含量小于3.0 mg/kg）中的痕量金属卟啉的分子组成分析。除常见的钒卟啉ETIO外,鉴定了10个O2钒卟啉化合物,7个O3钒卟啉化合物和1个O2S1钒卟啉化合物,并实现了痕量钒卟啉和镍卟啉的同时分析,证实了在塔里木高成熟原油中存在金属卟啉化合物。本文还推测了O2钒卟啉、O2钒卟啉化合物和O2S1钒卟啉化合物及镍卟啉的地化形成机理。

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