现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (12) : 97-102. DOI: 10.16606/j.cnki.issn0253-4320.2025.12.017

科研与开发

低密度聚乙烯串联催化热解制芳烃过程研究

薛彦峰 ¹ ,
范旸杰 ¹ ,
刘馨宇 ¹ ,
郝晓栋 ¹^,² ,
李嘉诚 ¹^,²

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Study on production of aromatics from cascade catalytic pyrolysis of low-density polyethylene

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摘要

采用两段式固定床,研究了低密度聚乙烯(LDPE)在固体酸催化剂(HY、HZSM-5和Al₂O₃)上热解后,尾气经Zn/ZSM-5进行芳构化的过程,探讨了热解催化剂类型、热解温度及原料混合比例等条件对产物分布的影响。结果显示,在热解催化剂为ZSM-5时油相选择性和BTX选择性最高,分别达到58.9%和66.4%。提高热解催化剂的用量可显著提升油相和BTX选择性,在m(LDPE)∶m(HZSM-5)=1∶1时所得油相和BTX选择性分别达到64.0%和69.8%。

Abstract

The pyrolysis of low-density polyethylene (LDPE) over solid acid catalysts (HY,HZSM-5,and Al₂O₃),and then aromatization of the exhaust gas from pyrolysis are studied over Zn/ZSM-5 catalyst in a two-stage fixed bed.The influences of the types of pyrolysis catalyst,pyrolysis temperature,and the raw material mixing ratio on products distribution are investigated.The results show that ZSM-5 as the pyrolysis catalyst delivers the highest oil phase selectivity and BTX selectivity,reaching 58.9% and 66.4%,respectively.The higher dosage of pyrolysis catalyst leads to an improvement selectivity for both oil phase and BTX.The selectivity of oil phase and BTX are 64.0% and 69.8%,respectively when m(LDPE)∶m(HZSM-5)=1∶1.

Graphical abstract

关键词

低密度聚乙烯 / 废塑料 / 芳构化 / 热解 / 串联催化

Key words

low-density polyethylene / waste plastics / aromatization / pyrolysis / cascade catalysis

Author summay

薛彦峰(1981-),男,博士,副教授,研究方向为分子筛催化, xueyf@tit.edu.cn。

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据统计,2020年全球塑料生产量已高达9.2亿t^[1],每年产生的废旧塑料已超过2.58亿t^[2]。大量废塑料被焚烧、填埋或丢弃在自然环境中,由此产生温室气体排放、土壤、水污染等环境问题^[3]。芳烃类化合物如苯、甲苯和二甲苯(BTX)是重要的基础化工原料,主要来自石脑油催化重整和石油裂解等^[4]。在当前石油资源日益短缺且芳烃需求旺盛的背景下,废塑料制备芳烃路线不仅有望实现废塑料的资源化和清洁化利用,也能促进芳烃的稳定生产,因此该路线受到人们的广泛关注。

聚乙烯、聚丙烯等废塑料可通过热裂解或催化热解等方式,经氢转移、环化、脱氢、异构化等反应转化为烷烃、烯烃和芳烃等产物^[5-6]。常见的废塑料热解催化剂包括分子筛^[7]和金属氧化物^[8]等,其中,分子筛因独特的酸性和对产物择形能力在废塑料催化热解制芳烃中表现出一定的优势。例如,Zhang等^[9]采用ZSM-5为催化剂,在450℃、微波辅助条件下研究了LDPE催化热解制芳烃过程,油相产率达到32.58%,油相中单环芳烃占比达到74.73%以上。Fu等^[10]研究了线性LDPE在Ga/ZSM-5分子筛上的催化热解行为,发现热解油产率和芳烃选择性可达到51.25%和94.02%。Tian等^[11]比较了HZSM-5、Hbeta、HY和MCM-41分子筛催化LDPE制芳烃的催化性能,发现ZSM-5上单环芳烃选择性最高,达到了84.46%。

为实现芳烃收率的增加,研究者们提出了分段串联催化热解制芳烃思路^[10-12],将废塑料首先在第一段反应器进行热裂解或催化热解,随后热解产物进入到第二段进行芳构化反应。采用分段式系统可以对不同反应段的反应温度、催化剂种类等进行单独调控,实现对反应更科学的优化。例如,Duan等^[13]报道了串联催化系统用于废聚乙烯催化转化制取芳烃,其C₅+产物收率达到60.1%,芳烃选择性达到76.7%。值得注意的是,在串联系统中如何解决废塑料热解和芳构化反应的高效耦合匹配仍具有一定的挑战性。

基于以上分析,本文中以LDPE为废塑料模型化合物,采用分段串联催化方式,对其热解-芳构化行为进行了研究,探讨了热解催化剂类型、热解温度、原料混合比例等对油相产率及芳烃选择性等的影响。

1 实验部分

1.1 催化剂制备

ZSM-5分子筛采用晶种法制备^[14]。采用硅溶胶(JN-25,青岛海湾精细化工有限公司)为硅源,偏铝酸钠(NaAlO₂,上海阿拉丁生化科技股份有限公司)为铝源,四丙基氢氧化铵(TPAOH,40%,肯特催化材料股份有限公司)为模板剂,配制组成为1 SiO₂∶0.0056 Al₂O₃∶0.05 NaOH∶0.15 TPAOH∶25 H₂O的合成凝胶,同时加入1%的晶种,搅拌均匀后转移至100 mL水热合成釜中,在170℃下晶化48 h,产物经离心、洗涤、干燥后,于560℃焙烧10 h,得到Na-ZSM-5。使用1 mol/L氯化铵溶液对Na-ZSM-5进行离子交换2次,得到NH₄-ZSM-5。经干燥,550℃焙烧后得到HZSM-5。HY分子筛(Si/Al=2.7)购买于南开大学催化剂厂。Al₂O₃催化剂采用煅烧法制备。将拟薄水铝石(高纯低钠型,德州市晶火技术玻璃有限公司)放置于马弗炉中,在550℃焙烧 2 h得到Al₂O₃催化剂。Zn/ZSM-5催化剂(Zn质量分数1%)采用浸渍法制备。称取一定量六水合硝酸锌溶于水,将HZSM-5分子筛(Si/Al=36)加入硝酸锌溶液中,超声震荡5 min后静置12 h,随后经烘干,550℃焙烧5 h后得到Zn/ZSM-5。

1.2 催化剂表征

X射线粉末衍射(XRD)表征在日本Rigaku公司MiniFlex Ⅱ X射线衍射仪上进行,测试条件为:Cu靶Kα辐射(30 kV,15 mA),扫描速率为8(°)/min,5°~45°扫描。热重分析在梅特勒托利多公司TGA-DSC 3+热重分析仪上进行。在氮气气氛下以 10℃/min升温至800℃,对其热重及热流数据进行检测。氨气-程序升温脱附表征(NH₃-TPD)在美国Micromeritics公司的AutoChem Ⅱ 2920型化学吸附仪上进行。以10℃/min升温至600℃,采用热导池检测器对脱附的氨气进行检测。采用北京精微高博仪器有限公司的JW-TB440A型比表面积及孔径分析仪对样品进行N₂物理吸附表征,在-196℃下测定样品的N₂吸附-脱附等温线。

1.3 催化剂评价

使用两段式固定床反应装置进行LDPE串联催化热解制芳烃实验,具体装置见图1。反应装置包括一段反应炉(热解段)、二段反应炉(芳构化段)、冷凝器、尾气计量及收集、气相色谱分析和温度/气体流量控制装置等组成。实验前,将一定量LDPE(上海麦克林生化科技股份有限公司,熔融指数 20~30 g/10 min)和热解催化剂充分混合后进行压片、造粒,筛选出粒径为20~40目的颗粒样品。将 5 g颗粒样品放置于反应器热解段恒温区,将0.5 g Zn/ZSM-5催化剂放置于二段恒温区,采用N₂(50 mL/min)作为反应载气。将二段炉加热到预定温度450℃,随后将一段炉以20℃/min升温至反应温度(450~600℃)并保持3 h。产物经冷阱(0℃)分离后进入湿式流量计计量,随后采用气袋收集尾气。待反应结束后,采用气相色谱仪(北京东西电子GC-4100,色谱柱为KB-PLOTQ毛细管柱,FID检测器)对尾气进行分析。油相产物采用浙江福立GC 9790 PLUS型气相色谱仪分析,色谱柱为安捷伦HP-PONA毛细管,配备FID检测器。

油相产率(Y)和产物选择性(S_i)按式(1)、(2)进行计算:

(1)

Y = (油 相 质 量 / L D P E 装 填 质 量) × 100 %

(2)

S i = (产 物 中 i 组 分 所 含 碳 摩 尔 数 / 所 有 产 物 中 总 碳 摩 尔 数) × 100 %

2 结果与讨论

2.1 热解与芳构化催化剂的基本理化性质分析

图2为催化剂样品的XRD谱图。HZSM-5和Zn/ZSM-5样品表现出典型的MFI拓扑结构衍射峰^[15],HY样品则表现出FAU拓扑结构特征衍射峰^[16],且均未观测到其他额外的杂质峰。Al₂O₃样品的特征衍射峰强度相对较弱,在2θ=36.8°和45.9°处表现出γ-Al₂O₃物种的衍射峰^[17]。

采用氮气物理吸附对催化剂的孔结构进行分析,结果如图3和表1所示。HY、HZSM-5和Zn/ZSM-5样品在低压区N₂吸附量上升明显,说明3种催化剂中存在明显的微孔结构,同时在p/p₀>0.8时出现H4型回滞环,说明3种样品上存在一定量的介孔^[18]。Al₂O₃样品在0.5<p/p₀<1范围内有非常明显的迟滞回环,说明该材料中存在大量的介孔结构,图3(b)显示其孔径范围覆盖2~100 nm。从表1可知,HY样品比表面积最大,达到433 m²/g。尽管Al₂O₃样品的比表面积最小,为173 m²/g,但总孔体积和平均孔径较大,达到了0.38 cm³/g和8.84 nm。

图4为样品的NH₃-TPD谱图。HZSM-5、HY和Zn/ZSM-5样品出现2个明显的氨气脱附峰,分别对应于弱酸和强酸中心的脱附峰^[19]。Al₂O₃样品在200℃附近出现一个宽泛的弱酸峰,该峰向高温方向延展,说明样品上除弱酸外,还存在一定数量的中强酸。表1列出了经定量后得到的酸量结果。HY分子筛总酸量最高,达到1.91 mmol/g。Al₂O₃和HZSM-5分子筛酸量接近,但前者强酸的酸强度明显弱于后者。

2.2 LDPE在不同固体酸材料上的热解行为分析

采用热重分析仪对LDPE和热解催化剂的混合样品进行了分析,结果如图5所示。LDPE+HZSM-5样品在250℃左右开始快速失重,最大失重率出现在404℃。LDPE+HY样品在290℃后开始快速失重,失重速率在403℃达到最大。LDPE+Al₂O₃样品在390℃后开始快速失重,最大失重率出现在478℃。从NH₃-TPD结果可以看到,Al₂O₃样品上强酸酸强度明显弱于HZSM-5和HY,这应是导致LDPE+Al₂O₃样品热解温度偏高的原因,说明提高热解催化剂的酸强度可以降低LDPE的热解温度。3个样品在250~500℃范围内总失重率为78.3%~81.2%,均接近LDPE在混合样品中质量分数(80%),说明LDPE在这3种热解催化剂上热解完全,几乎不存在热解残余物。

图5(c)为混合物在热重分析中的差示扫描量热曲线。3种样品在104~132℃均存在一个较小的吸热峰,对应于样品上水分的蒸发。第二个吸热峰则出现在272~285℃,可能是LDPE由固体向熔融态晶相转变时吸热导致^[20]。随温度升高,LDPE+HY、LDPE+HZSM-5和LDPE+Al₂O₃分别在388、407、451℃附近出现明显的吸热峰,与图5(a)对照可知,此时3种样品均处于快速失重期,因此该吸热峰应是LDPE分子发生C—C键、C—H键断裂时吸热导致。随后3种样品在447~515℃范围内表现出一个放热峰,这可能是由于LDPE分子裂解产物在酸性催化剂上发生二次反应(如环化、芳构化等)放热导致。

2.3 LDPE催化热解制芳烃反应性能

2.3.1 LDPE单段催化热解与两段催化热解过程对比

为明确芳构化段Zn/ZSM-5催化剂在LDPE转化制芳烃的促进作用,首先对LDPE单段与两段串联催化热解过程进行了对比。热解段反应温度均控制在550℃,产物组成见表2。单段式油相产率为15.4%,两段式则增加至38.0%。在气相组成中,串联式的C₁~C₄烷烃和C₂~C₄烯烃选择性明显低于单段式,可见低碳烷烃和烯烃在Zn/ZSM-5催化剂上进一步进行了芳构化反应。油相中BTX选择性由单段时28.0%增加至两段时66.4%,进一步证实该推测。此外,油相产物中C₆+非芳香烃选择性由单段式时50.7%下降至两段式时13.0%,说明在Zn/ZSM-5催化剂存在下,长链烷/烯烃、环烷/烯烃等物质可能进一步发生环化、脱氢等,从而促进芳烃产物的生成。以上结果证实,两段式串联反应体系对于LDPE催化热解过程的产物提质有显著的促进作用,芳烃选择性有明显提升。

2.3.2 热解催化剂类型对LDPE串联催化热解制芳烃产物分布的影响

表3为不同热解催化剂上LDPE制芳烃的产物分布。采用HZSM-5为热解催化剂时,产物中油相选择性最高,达到了58.9%。Al₂O₃上油相产物选择性最低(47.1%),但其C₁~C₄烷烃选择性较高,为30.6%。在HZSM-5的油相产物中,BTX选择性达到66.4%,远高于HY(41.1%)和Al₂O₃(41.8%)。但HY和Al₂O₃上C₆+非芳香烃的选择性却达到43.8%和36.9%,远高于HZSM-5(13.0%)。结合NH₃-TPD结果分析,尽管ZSM-5上总酸量仅为HY的15.7%,但其油相和BTX选择性均超过了前者,说明HY分子筛上C₆+非芳香烃并非因缺少足够的酸性位而未发生芳构化。同时,尽管HZSM-5和Al₂O₃的总酸量接近,但产物分布尤其是BTX选择性表现出较大的差异。可见除酸性外,分子筛拓扑结构、孔道尺寸等亦对其产物选择性具有重要的影响。相比而言,HZSM-5分子筛因适中的酸性和孔道尺寸表现出最高的BTX选择性。

2.3.3 热解温度对LDPE串联催化热解制芳烃产物分布的影响

表4为热解温度对LDPE串联催化热解制芳烃产物分布的影响。随着热解温度从450℃升高至600℃,油相选择性呈现出先增加后降低的趋势,在500℃时油相选择性最高,达到59.5%。在550℃时BTX选择性最高,达到66.4%。整体上看,热解温度对产物分布的影响并不显著,可能是由于不同热解温度下得到的尾气进入到芳构化段后,在Zn/ZSM-5上重新发生二次反应,最终产物的差异性并不明显。

2.3.4 原料混合比例对LDPE串联催化热解制芳烃产物分布的影响

表5为LDPE与HZSM-5混合质量比例分别为1∶1、2∶1和4∶1时的产物组成。随着LDPE占比的提高,油相选择性随之下降,C₁~C₄烷烃和C₂~C₄烯烃选择性逐渐提高。在油相组成中,BTX选择性由1∶1时69.8%降低至4∶1时的65.2%,C₆+非芳香烃选择性由6.6%提高至14.3%。在1∶1时,混合物中HZSM-5占比相对较高,为LDPE裂解及后续芳构化提供了丰富的酸性位点,因此低碳烷烃、烯烃及C₆+非芳香烃等选择性均低于4∶1时,应是这些产物在酸性位作用下进行了后续的芳构化反应,从而生成了更多的芳烃。

3 结论

采用HY、HZSM-5和Al₂O₃为热解催化剂,Zn/ZSM-5为芳构化催化剂,开展了LDPE经串联催化制备芳烃的研究。串联催化热解和单段式热解产物对比表明,串联过程能显著提升油相产率和芳烃选择性。当选用不同热解催化剂时,HZSM-5表现出最佳的催化性能,油相选择性和油相中BTX选择性分别达到了58.9%和66.4%。此外,热解段反应温度对产物分布的影响并不显著,但LDPE与热解催化剂(HZSM-5)不同混合质量比会显著影响产物分布。当LDPE与HZSM-5混合质量比例为1∶1时油相选择性和BTX选择性最高,分别达到64.0%和69.8%。

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