“金属氧化物/分子筛” 新类型活性相的制备及轻烃催化裂解性能

The precess of catalytic cracking naphtha to olefins has gained much attention, due to it producing high yield light olefins, especially propylene and reducing energy cost and CO2 emission. Usually, the naphtha cracking over catalyst takes place at temperature of 600-650℃ and the steam is used as an inexpensive diluent of naphtha during the catalytic cracking for the decrease of the coke deposition. However, the catalysts are also deactivated by the dealumination of ZSM-5 in the case of catalytic steam cracking. The deactivated catalyst by the coke deposition can be regenerated by calcination; however, the dealumination of ZSM-5 is irreversible. Therefore, the deactivation of ZSM-5 catalysts is an issue of the greatest concern inacademic and commercial olefin production from naphtha. However, it is still a challenge to synthesize zeolites with a stable acidity but without reducing the number of active sites, because it is necessary to attain a high paraffin conversion in the catalytic cracking process. .Herein, a novel concept and method of building“metallic oxide/zeolite”one phase composite (OX/ZEO OPC) is proposed and used to enhance the surface acidity of zeolite and structure stability for cracking naphtha. Typically, an assembly method is developed by forming metallic oxide cluster via a certain technology, then it is fixed into ZSM-5 bulk to produce the OXE/ZEO OPC material. The OPC material containing metal oxide and zeolite, maintains one phase crystal structure, simultaneously bearing the properties of both zeolite and oxide, thus the issue of unstability of oxide clusters will be overcome due to the strong interaction of oxide clusters and zeolite surface. Thus, high catalytic performance will be achieved because of the existence of two types of active sites and their synergism. However, further elucidation of the interplay between the OXE/ZEO OPC structure (surface structure/acidity) and the activity/selectivity/deactivity will require much more experiments..(1) Study of the synthesis of OX/ZEO OPC, including active metal oxide precursor ( for example, MnOx and ZSM-5 is the best choice) and technological parameters;.(2) Understand the interpaly between OXE/ZEO OPC microscopic structure and its catalytic performance, especially, the effect of oxide chemical behaviour, acidic sites variation in the OX/ZEO OPC ZSM-5 material, in combination with its intrinsic diffusion on its catalytic performance; .(3) Theoretically, microstructure of the catalyst including behaviour of the metallic oxide cluster，surface charicteristics, pore mophology, spatial structure is investigated by Synchrotron Radiation X-ray Absorption Fine Structure (XAFS) analysis, Small Angel X-ray Scattering experiments and other thchniques, which will improve the development of zeolite microstructure but also deep understanding the naphtha cracking reaction and its deactivity behaviour.

轻烃催化裂解技术已成为取代传统热裂解工艺的主流方向和定位。解决分子筛在裂解反应过程中面临的失活问题，尤其是分子筛的不可逆脱铝失活是关键。本项目创新地提出“金属氧化物/分子筛”单相复合体（OX/ZEO）新类型活性材料制备与应用研究，即通过一定工艺将几个原子的金属氧化物簇有序定位在ZSM-5分子筛表面，并形成稳定的单相晶相物质，这种结构既保持分子筛的特点，又解决了簇的不稳定性问题。这种将两类催化材料有机组合，将金属化合物和分子筛两类材料的功能提升为最佳状态，为分子筛酸性位调变及提高结构稳定性提供新思路。结合先进的同步辐射技术（XAFS、SAXS），建立（OX/ZEO）单相复合体微观-介观结构表征方法，研究其在轻烃裂解反应和失活行为的构效关系，揭示新类型催化剂的酸性调变及影响分子筛结构稳定的作用机制，将催化裂解催化剂研究水平提升到新高度。为轻烃催化裂解工艺关键催化剂的研制及工业化奠定基础。

ZSM-5分子筛由于具有独特的孔道结构、可调变的酸性和高水热稳定性在通常被认为是催化裂解、裂化、苯烷基化等固体酸催化反应中优选的催化活性材料。如何提高催化剂的活性及减少催化剂失活是分子筛催化反应中普遍问题。通常采用对ZSM-5分子筛进行改性，尤其是金属改性分子筛作为提高ZSM-5分子筛的催化性能的手段，但催化剂应用中面临的挑战是解决转化能力提高与降低积炭失活的难题。本项目提出金属氧化物/分子筛复合体的制备，采用一定的工艺将纳米金属氧化物在ZSM-5上形成金属氧化物簇制备金属氧化物簇/分子筛复合体，实现对酸性结构的调变作用和提高酸性稳定性，提高其在烷烃裂解转化活性、提高低碳烯烃选择性，降低分子筛的积炭从而达到提高催化剂的稳定性的目的。.采用纳米氧化铜前驱体对ZSM-5分子筛改性，研究了其微结构的影响、酸性质的变化和反应性能的影响。同时与传统铜盐溶液在分子筛浸渍、离子交换的方法对比研究。结果表明，在正己烷催化裂解反应性能中(630℃，空速8h-1 )，氧化铜改性方法其双烯收率提高3-5个百分点，反应寿命是改性前分子筛的8倍以上；采用新方法氧化铜改性样品的积炭量为传统方法的37%；研究表明新方法改性样品具有高的酸性稳定性，水热后强酸保留度提高大幅提高。在高温水热条件下，氧化铜纳米离子分解为CuOx簇与ZSM-5分子筛晶体内缺陷羟基结合，形成CuOx/ZSM-5复合体。与传统金属改性方法不同，CuOx/ZSM-5复合体上形成了新的Lewis酸性位，而分子筛强Brønsted酸性位没有降低；CuOx/ZSM-5分子筛复合体也稳定了CuOx簇的结构，因此，CuOx/ZSM-5复合体形成分子筛的酸性稳定的提高，也保护了骨架Al的脱除；结合同步辐射XAFS研究CuOx在分子筛表面的化学行为研究，在反应及再生后的复合体催化剂上几乎没有金属的结焦且极少的金属流失。上述因素的共同作用下，CuOx/ZSM-5分子筛复合体在催化己烷裂解反应中获得了高的活性、低碳烯烃选择性和低的积炭量，从而延长催化寿命。.采用氧化锰及其它金属氧化物，得出同样类似的结果，说明该创新方法具有一定的普适性。将进一步研究其在其它分子筛中的改性作用，并开拓分子筛在烃类裂解、甲醇转化制化学品、芳烃烷基化等酸催化反应中应用，将会取得显著的效果。