【作者】 金涛；

【导师】 定明月；

【作者基本信息】 武汉大学 ， 化学工艺， 2022， 硕士

【摘要】 生物质是巨大的、可再生的资源和能源宝库,其储量丰富、分布广泛,如实现规模化应用可有效助力我国“碳达峰、碳中和”战略目标的实现。因此,将生物质催化转化为能源燃料的技术被认为具有很好的发展前景和研究意义。近年来,生物质快速催化热解制备高附加值化学品的技术得到了越来越多的关注,因为苯、甲苯、二甲苯(BTX)等单环芳烃作为该反应的主要产物,是重要的化工原料和燃料。目前,单环芳烃的主要生产原料仍然是石油等不可再生的传统化石资源,其过度的开发给环境带来沉重的负担。生物质的快速催化热解为单环芳烃的生产提供了另外一条途径。HZSM-5分子筛(一种多孔固体酸催化剂)能将生物质热解产物催化脱氧转化为芳香烃。然而,商用HZSM-5的催化活性还有待加强,需改进的性质包括其酸性、孔道可达性、比表面积等。本文通过化学处理法、原位合成法、金属负载法等手段获得了不同孔道性质和酸性性质的HZSM-5分子筛,并通过表征技术和木质素快速催化热解实验来揭示催化剂的构效关系,大大提升了热解产物向单环芳烃的转化效率。首先,研究了有机碱(TMAOH、TEAOH、TPAOH和TBAOH)在一种长链有机铵(CTAB)的辅助下处理商用HZSM-5以扩大其孔道,并探究了碱种类、碱浓度和CTAB添加量对处理后分子筛结构性质的影响。未经处理和碱处理后的分子筛由多种表征技术分析,并用于木质素快速催化热解过程。结果显示,由CTAB辅助的0.5 mol/L的TPAOH处理后的HZSM-5分子筛具有非常合适的介孔和微孔比例、恰当扩大的平均介孔孔径和保留较好的酸性位点数量。足量CTAB的存在不仅能防止碱对分子筛的过度处理,还能调控生成的介孔结构,进而调控催化产物的分布。处理后催化剂的催化性能提升明显,其生成的单环芳烃含量从17.3%(商用HZSM-5)最高可提升至31.8%。然后,利用绿色、节能的原位合成技术(动力学控制结晶法)合成了与商用分子筛形貌不同的微介孔HZSM-5,并重点通过调变三类合成条件(微孔结构导向剂TPAOH的添加量、低温水热反应的温度、高温水热反应与低温水热反应的时长)来优化该微介孔HZSM-5分子筛的孔道结构、酸性性质和催化生成单环芳烃的性能。研究发现,相较于商用HZSM-5分子筛,原位合成的纳米微介孔分子筛具有更小的粒径、更大的介孔。这些由尺寸很小的晶粒堆积生长而成的催化剂颗粒形成的多级孔道结构可有效降低传质阻力,进而利于催化反应的高效进行。当水热温度较高或高温水热阶段较长时,合成催化剂的酸性更强,但晶粒偏大,传质阻力偏大,催化效率下降。当合成条件为110℃水热1天,170℃水热两天时,催化剂的酸性和结构性质达到了最佳的平衡,催化生成单环芳烃的含量达到39.3%。另外,在获得了较好的孔道结构之后,还继续对催化剂进行了1%质量的金属(Fe、Co、Ni、Cu、Zn、Ga)负载改性,以进一步优化催化剂的催化活性。对于商用HZSM-5,Co的负载能同时增加其酸量、介孔比表面积和介孔孔容,这最终大幅提升了其催化生成单环芳烃的性能(从17.3%上升到27.6%)。另外,1%Co的负载对于碱处理后的分子筛和原位合成的分子筛的催化性能也有明显的提升,分别从31.8%、39.3%提升至36.9%、44.0%。随后调变了Co在合成分子筛上的负载量,发现当Co的负载量增加时,其介孔比表面积同步增加,而微孔比表面积下降。总的比表面积在1%的负载量下达到最大值,此时Co对孔道的堵塞作用也很轻微,而对酸性有较明显的提升,这也使得1%Co负载的合成分子筛具有最高的催化单环芳烃生产的性能。最后对催化区的反应温度进行了优化,发现升高温度会同时加强中间产物的转化和多环芳烃副产物的生成,最终在550℃条件下得到了最高的单环芳烃含量(46.3%)。所筛选出的最佳催化剂(1%Co负载的原位合成多级孔HZSM-5)在10次再生循环利用后表现出很好的稳定性,表明其可重复使用性高。更多还原

【Abstract】 Biomass is a huge treasure house of renewable resource and energy with abundant reserves and wide distribution.The realization of large-scale application of biomass can effectively promote achieving the strategic goal of carbon peaking and carbon neutralization of our country.Hence,catalytic transformation of biomass into biofuels has been considered very promising and meaningful.Catalytic pyrolysis of biomass has attracted tremendous attention these years because the primary products are monocyclic aromatic hydrocarbons(MAH,such as benzene,toluene,and xylene),which are significant raw materials and fuels and are mainly produced using non-renewable traditional fossil resources,which brings a heavy burden to the environment.The catalytic fast pyrolysis of biomass provides an alternative way for the production of MAH.HZSM-5 zeolite(a kind of porous solid acid catalyst)can catalyze the deoxygenation reaction,converting biomass decomposition products into aromatic hydrocarbons.However,the catalytic activity of commercial HZSM-5 needs to be strengthened by improving its acidity,pore accessibility,specific surface area and so on.In this paper,HZSM-5 zeolites with different pore structure and acidity were obtained by means of chemical treatment,in-situ synthesis and metal modification.The structure-performance relationship of the catalysts was revealed by characterization technologies and catalytic pyrolysis experiments of lignin,which greatly improved the conversion efficiency of pyrolysis products to MAH.Firstly,the treatment of commercial HZSM-5 with organic alkalis(TMAOH,TEAOH,TPAOH and TBAOH)assisted by a long-chain organic ammonium salt(CTAB)was carried out to expand its pores,and the effects of alkali type,alkali concentration and CTAB addition on the structural properties of the treated zeolite were explored.The untreated and alkali treated samples were analyzed by various characterization techniques and applied in catalytic pyrolysis of lignin.As it turned out,the HZSM-5 zeolite pretreated by 0.5 mol/L of TPAOH in the presence of CTAB exhibited a coordinated micropore/mesopore proportion,proper average mesopore size,and a successful preservation of acidic properties.The presence of sufficient CTAB in the alkali treatment process was not only beneficial to preventing the over-treatment of HZSM-5,but could also regulate its mesoporous structure.The catalytic performance of the treated catalyst increased obviously,and the content of MAH could be elevated up to 31.8%(17.3%obtained by commercial HZSM-5).Then,the micro/mesoporous HZSM-5 zeolites with different morphology from the commercial zeolite were synthesized via the green and energy-saving in-situ synthesis technology(controlled crystallization method),and the pore structure,acidity and catalytic performance were optimized by regulating three synthetic conditions(amount of TPAOH,the temperature and duration of hydrothermal reaction process).Compared to commercial HZSM-5,the synthesized zeolites had smaller particle size and larger mesopores.The hierarchical pore structure formed by the accumulation and growth of small crystals could effectively lower the mass transfer resistance,being beneficial to the efficiency of catalytic reaction.When the hydrothermal reaction temperature was relatively high or the high-temperature hydrothermal process was relatively long,the synthesized zeolite exhibited stronger acidity,but a larger particle size,which led to higher mass transfer resistance and inferior catalytic performance.When the zeolite was synthesized under the hydrothermal reaction condition of 110℃ for one day and 170℃ for two days,the acidity and structural properties of the catalyst was best balanced,and the content of MAH reached 39.3%.In addition,the micro/mesoporous catalysts were further modified by metal(Fe,Co,Ni,Cu,Zn,Ga)to optimize the catalytic activity.For commercial HZSM-5,the loading of Co could simultaneously increase its acid amount,mesoporous specific surface area and mesoporous volume,eventually improving its catalytic performance(MAH production)from 17.3%to 27.6%.Besides,the introduction of 1%Co could also obviously enhance the catalytic performance of alkali treated or in-situ synthesized zeolite samples,from 31.8%and 39.3%to 36.9%and 44.0%,respectively.Afterwards,the loading amount of Co on the synthesized zeolite was adjusted.As it turned out,when increasing the loading amount of Co,the mesoporous specific surface area increased synchronously,while the microporous specific surface area decreased.The total specific surface area reached the maximum at 1%Co loading.Inappreciable pore blockage and obvious enhancement of acidity were observed,making this 1%Co modified zeolite the most effective one in MAH production.Moreover,the catalytic reaction temperature was optimized.It was found that increasing the temperature would favor both the conversion of intermediate products and the formation of by-products(polycyclic aromatic hydrocarbons),and the content of monocyclic aromatic hydrocarbons reached the highest value of 46.3%at 550℃.The screened-out catalyst(in-situ synthesized hierarchical HZSM-5 with 1%Co loading)showed great stability even after 10 regeneration cycles,indicating the high reusability.更多还原