不同晶粒尺寸H-ZSM-5催化剂上甲醇芳构化反应性能

芳烃是一类重要的有机化工基础原料,通常采用传统的石油路线生产芳烃,包括催化裂化和催化重整等工艺.由于石油资源的紧缺,以可再生资源为原料生产芳烃工艺的发展具有十分重要的意义.甲醇作为一种重要的基础原料,可来源于煤、天然气和生物质等,因此,甲醇制芳烃工艺(MTA)的研究受到日益关注.ZSM-5分子筛具有较大的比表面积、可调节的酸性、优良的择形选择性和很高的水热稳定性,因而在甲醇芳构化中展现出良好的催化性能.研究发现,甲醇转化率和产物分布与ZSM-5分子筛的酸性和多孔性等密切相关.本文通过调控模板剂与水的比例和晶化时间,采用水热法制备了一系列不同晶粒度H-ZSM-5分子筛催化剂,通过X射线衍射(XRD)、扫描电镜(SEM)、N2物理吸附脱附(BET)和X射线荧光光谱等技术对所得分子筛的理化性质、骨架结构和形貌进行了表征;采用吡啶红外光谱和NH3程序升温脱附技术对其酸性进行了分析,使用热重(TG)技术对反应后催化剂的积碳含量进行了分析,并将所制备的H-ZSM-5分子筛催化剂分别应用于MTA反应,系统性地探究分子筛晶粒度对其理化性质和MTA催化性能的影响.XRD结果表明,所合成的五种样品均具有典型的ZSM-5分子筛特征衍射峰且无杂晶,且具有不同的晶粒度,分别为4.0±0.3,1.2±0.2 μm,614.1±31.9、391.9±32.4和99.1±7.0 nm.N2物理吸附脱附曲线可以发现,晶粒度为99.1±7.0 nm的ZSM-5分子筛展现出典型的Ⅰ型和Ⅳ型物理吸附曲线且在较高的相对压力(P/Po=0.8-1.0)处有一个明显的H4型迟滞环,表明此分子筛具有介孔和大孔结构;BJH吸附孔径分布图表明,这些介孔主要分布在2-7和20-50 nm范围内;同时各样品的比表面积和孔体积随着其晶粒度的减小而增大.结果还表明五种不同晶粒度的ZSM-5分子筛具有相似的SiO2/Al2O3摩尔比和酸性质.MTA反应结果表明,随着催化剂晶粒度的降低,甲醇的平均转化率,芳烃选择性和BTX选择性有所提高,在300 min时晶粒度较大的三个催化剂上,甲醇转化率迅速降至90％,而晶粒度较小的两个催化剂上,甲醇转化率始终维持在95％以上,其中晶粒度为99.1±7.0 nm的样品上芳烃选择性最高(平均42％以上),BTX选择性达37％.对失活催化剂积碳含量分析,随着催化剂晶粒度的降低,积碳量降低.晶粒度较低的纳米分子筛催化剂具有更短的孔道,更高效的扩散性能,更高的比表面积和独特的梯级孔结构,因而在甲醇芳构化反应中展现出更长的寿命,更高的活性和更低的积碳量,在甲醇制芳烃工业化生产中具有巨大潜力.

Correlation between H-ZSM-5 crystal size and catalytic performance in the methanol-to-aromatics reaction

The porosity of H-ZSM-5 zeolite is known to facilitate the diffusion of molecules in the methanol-to-aromatics (MTA) reaction.The activity and selectivity of the H-ZSM-5 catalyst in the MTA reaction has been studied as a function of crystal size.ZSM-5 zeolites with different crystal sizes were successfully synthesized by conventional hydrothermal methods.Tailoring ZSM-5 particle size was easily controlled by changes to the sol-gel composition,and in particular,the deionized water to tetrapropylammonium hydroxide ratio,and crystallization time.The structure of the H-ZSM-5 zeolites were characterized by X-ray diffraction and the morphology of the zeolite particles was determined by scanning electron microscopy.N2 adsorption-desorption measurements established changes to the textural properties,and compositional properties were characterized by X-ray fluorescence spectroscopy.Acidity measurements of the catalysts were measured by pyridine-adsorbed Fourier transform infrared spectroscopy and the temperature-programmed desorption of ammonia.After subjecting the catalysts to the MTA reaction,the total amount of coke formed on the spent deactivated catalysts was determined by thermal gravimetric analysis.The results show that the SiO2/Al203 molar ratios and acidic properties of the H-ZSM-5 samples are similar,however,the nano-sized hierarchical ZSM-5 zeolite with an additional level of auxiliary pores possesses a higher surface area,larger mesopore volume and larger macropore volume,resulting in more effective mass transportation properties.The H-ZSM-5 zeolite catalysts were evaluated for their activity towards the MTA reaction as a function of crystal size.The nano-sized H-ZSM-5 catalyst having shorter diffusion path lengths,substantial mesoporosity and a high external surface area showed excellent selectivity toward aromatics (average ＞ 42％) and benzene,toluene and xylenes (37％ at 180 min).Furthermore,lower coking levels were observed on the nano-sized H-ZSM-5 catalyst when compared with the H-ZSM-5 catalysts possessing larger particle sizes and is attributed to a reduction in polyaromatic hydrocarbons being deposited within the internal surface area.