Study On Catalytic Properties Of HZSM-5in LPG Aromatization

Liquefied petroleum gas (LPG) is a kind of traditional civil fuel, which isgradually being replaced by natural gas. The utilization and development ofLPG is considerable significance. In recent years, LPG aromatizationtechnology provides a new opportunity for the development and utilization ofLPG. The objective products of LPG aromatization are benzene, toluene,ethylbenzene and xylene (BTEX), which have not only been the most necessaryraw materials in the chemical industry, but also may be transformed intohigh-octane gasoline. Therefore, LPG aromatization technology is contributed toachieve the reuse of LPG and improved the supply of aromatics. HZSM-5isused as catalyst in LPG aromatization due to its three dimensional crossingstraight channel structure with ten-membered ring, large surface area, goodshape selective catalytic performance and special surface acidity. However, theHZSM-5showed short lifetime, which limited its large-scale industrialapplications.In this paper, we mainly investigated the conversion of the industrial LPGin a miniature fixed-bed evaluation device. The catalytic properties of differentnSiO2/nAl2O3ratio, alkali-treatment and Zn modification on HZSM-5+γ-Al2O3on the light paraffin aromatization were also investigated.Firstly, different nSiO2/nAl2O3ratios of HZSM-5+γ-Al2O3with38and50were modified by impregnation mothed to prepared Zn/(HZ+γ-Al2O3). Thenbased on the above optimal catalysts, a series of HZ-AT (t) and HZ-AT (c) wereprepared by alkaline treatment of NaOH aqueous solution with30min,120minand300min, then adding binder into the alkali-treated HZSM-5, and their catalytic activity was investigated to sieve the best alkali-treated time. Furtherbased on the best alkali-treated time catalysts, a series of HZ-AT(c) wereprepared by alkaline treatment of NaOH aqueous solution with0.1M,0.2M,0.3M and0.5M, then adding binder into the alkali-treated HZSM-5, and theircatalytic activity was investigated to sieve the best alkali solution concentration.Finally, the Zn material was impregnated to HZ-AT(t, c)+γ-Al2O3with Zn(NO3)2solution of0.13M,0.24M,0.46M and0.76M, their catalytic activity wasinvestigated to select optimal Zn concentration of catalysts. The syntheticcatalysts were further characterized by the XRD, MAS NMR, SEM, TEM,N2-absorption, NH3-TPD, Py-IR and TG. The conclusions were showed asfollowing:1) The XRD results show that the structure of HZSM-5catalysis is notsignificant affected by the addition of different nSiO2/nAl2O3ratio,alkali-treatment and impregnated Zn. All the catalysts remains a major MFIcrystalline structure. The MAS NMR results show that a small mountextra-framework Al atoms present were inserted into the framework atalkali-treated HZSM-5. Further, TEM and N2-adsorption results show smallamount of mesopores would be produced on the HZSM-5after alkali treatment,due to improving the surface area and adsorption pore volume. Py-IR andNH3-TPD results show that the Bronsted acid sites were decreased while theLewis acid sites were increased with the increase of alkali modification time andconcentration. And the amount and intensity of the strong acid catalyst is alsodecreased. Synergy effect of acid was the best when alkali-treated conditionswere0.2M NaOH for120min.2) Aromatization evaluation of HZ+γ-Al2O3and Zn/(HZ+γ-Al2O3) resultsshowed that catalysts of nSiO2/nAl2O3ratio of38exhibit the best aromatizationactivity, due to moderate weak acidic amount and strong acid amount. And theright ratio of weak and strong acidic amount, which enhanced hydrogen transfer alkanes, alkenes oligomerization, cyclization and aromatization capacity.3) Aromatization evaluation of alkali-treated catalysts by different time andconcentration results showed that the best catalytic stability and liquid yieldappear in HZ-AT(120,0.2) and HZ-AT(120,0.2)+γ-Al2O3catalysts. Becausemicropore surface area and pore volume were decreased while the specificsurface area and mesopore pore volume were increased with the increase ofalkali modification time and concentration. Thereby, increasing the carboncapacity of the catalyst, the catalyst lifetime is improved. And the right ratio ofweak, strong acidic amount and B, L acid amount, which enhanced aromaticsand liquid yield.4) Aromatization evaluation of Zn modified catalyst results showed that thebest activity appears in1.57%Zn/(HZ-AT(120,0.2)+γ-Al2O3) catalysts. Thestrong acid sites were decreased while the weak acid sites were increased withthe increase of Zn content, due to change the amount of total acid. Withincreasing of Zn content, the reduce amount of strong acid dued to weak initialactivation, while the increase amount of weak acid was due to enhancedehydrogenation and aromatization. Moreover, the amount of strong acid startedto experience decrease sharply with the continuing enhance of the Zn content,which depressed oligomerization and cyclization, further made yield ofaromatics decline. The synergistic effect of weak and strong acid was the bestwhen Zn content was about1.57%, which created to the highest aromatics yield.