Characterization and hydroisomerization performance of SAPO-11 molecular sieves synthesized in different media

Abstract

Silicoaluminophosphates SAPO-11 molecular sieves were synthesized from a conventional aqueous medium, a two-liquid phase medium and a HF medium, respectively, and were characterized by various methods such as XRD, SEM, pyridine-FT-IR, NH₃-TPD, ²⁹Si MAS-NMR. The morphologies of the SAPO-11 molecular sieves obtained from the conventional aqueous medium and the two liquid-phase media exhibit pseudo-spherical aggregates ranging from 7 to 10 μm assembled from cubic plate microcrystallites, whereas the SAPO-11 molecular sieves synthesized from the HF medium exhibit hexagonal prismatic aggregates of about 1–3 μm length. The synthesis media used highly influenced the number and the strength of SAPO-11 acid sites. Compared to SAPO-11 obtained from conventional aqueous medium, SAPO-11 prepared from two-liquid phase media possessed a similar number of BrØnsted acid sites but a much larger number of Lewis acid sites, whereas the SAPO-11 synthesized from HF medium had a similar number of Lewis acid sites but a much lower number of BrØnsted acid sites. The ²⁹Si MAS-NMR spectra showed that the HF could change the manner of Si incorporation into the SAPO-11 framework. The hydroisomerization of n-hexadecane on Pt/SAPO-11 samples showed that Pt/SAPO-11 obtained from the two-liquid phase media had the largest yield of isomers. The conversion activity of n-hexadecane was dependent on the amount of BrØnsted acid of SAPO-11 molecular sieves, while its hydrocracking activity was related to the strong acid sites of SAPO-11.

Introduction

Hydroisomerization of n-paraffins to branched paraffins is of considerable interest and plays an important role in the petroleum industry. The branching of n-paraffins is needed to improve the octane number of gasoline, to increase the low temperature performance of diesel and to obtain high viscosity index lube oils. Recently, the increasing demand for middle distillate products and the issuing of more stringent specifications pertaining to several characteristics (i.e. volatility, stability, cold flow properties, etc.) of automotive engine lubricants have raised the interest of refining industries in hydroconversion processes of heavy feedstocks and vacuum distillates to meet the market demand [1], [2]. Conventional lubricant dewaxing processes remove these normal paraffins by solvent extraction or by selective cracking, resulting in a yield loss directly proportional to the concentration of the normal paraffins in the oil. A more attractive dewaxing procedure, the so-called “isodewaxing” process, would result from catalytic dewaxing through isomerization of the n-paraffins to branched isoparaffins, eliminating the yield loss associated with n-paraffins removal by cracking or solvent extraction.

Currently, SAPO-11 has been found to be highly active and selective for the skeletal isomerization of n-butene [3], [4], especially for the hydroisomerization of long-chain n-paraffins [5], [6]. SAPO-11 crystal has the AEL structure and consists of non-intersecting elliptic 10-membered ring pores with 0.39 nm × 0.63 nm diameters. SAPO-11 is prepared under mild hydrothermal conditions from gels containing sources of aluminium, phosphorus and silicon and a structure-directing template such as dipropylamine. The desirable pH range of hydrous gel has been found to be 3.5–6.0 for successful crystallization of microporous silicoaluminophosphates. However, some other SAPO materials such as SAPO-5 or SAPO-31 have structures similar to that of SAPO-11, and the preparation of SAPO-11 is usually accompanied by the formation of the SAPO-5 and/or SAPO-31. To obtain pure SAPO-11, the chemical properties of the gel and the media for synthesis of SAPO-11 as well as the crystallization conditions are very important.

Bifunctional catalysts, using SAPO-11 as support and acid center and Pt or Pd as hydrogenation/dehydrogenation active sites, are known to be very effective in selective isomerization process of high freezing point waxes for production of high viscosity lube oils. Owing to the spatial constraints and low acidity of the bifunctional catalysts, the products obtained over it did not contain as many poly-branched isomers as those over the typical zeolite-based catalysts such as ZSM-5, ZSM-22, HY, HM and Hβ [7], [8], [9], [10]. For the hydroisomerization of long n-alkanes over bifunctional catalysts, the product distribution is determined by the average lifetime of the carbocation intermediates, which depends on the pore structure, topology, acid site density, acid strength and the metallic site to acid site ratio of the zeotype catalysts. For the SAPO-11 molecular sieves with a peculiar topology, the catalytic properties are strongly related to the nature of the acid sites in the framework. Both BrØnsted acid sites and Lewis acid sites can be observed on these solid acid catalysis materials. It is generally accepted that the conversion of n-alkane is highly dependent on the concentration of BrØnsted acid sites. However, the influences of the acid sites on the hydroisomerization reaction of long n-alkane were investigated on silicoaluminophosphate (SAPO) molecular sieves of difference topology, e.g., SAPO-11 (AEL), SAPO-31 (ATO), SAPO-41 (AFO), and ZSM series such as ZSM-23 (MTT), ZSM-22 (TON) [7], [11], [12], [13], [14], so that the “influences of the acid sites” may not only result from the acid sites, but also may come from the pore architecture of the molecular sieves. In addition, the role of Lewis acid sites was hardly mentioned, and the effect of acid strength on the hydroisomerization of n-paraffins is still in debate. Park and Ihm [7] attributed higher hydrocracking selectivity to strong acid sites of molecular sieves, whereas Nghiem et al. [13] and Hu et al. [14] reported that the acid strength of catalysts has little effect on isomerization selectivity and yield, and that the selectivity of hydroisomerization of long-chain alkanes depends mainly on the architecture of the pore channels of molecular sieves.

The acidity of SAPO materials is produced by silicon incorporation into hypothetical phosphorous T sites of the AlPO₄ framework. Thus, we can control the manner of silicon isomorphous substitution into the aluminophosphate framework to adjust the acidity of materials by using different synthesis methods. Li and coworkers [15] and Corma and coworkers [16] once obtained SAPO-11 molecular sieves in the presence of surfactants which could affect the physico-chemical characteristics of synthesized SAPO-11. Our previous work indicated that the addition of HF can increase the crystallization rate of SAPO-11and can change the acidity of SAPO-11 [17]. In the present work, we extended the work for synthesis of SAPO-11 in the presence of surfactant and HF, and concentrated on comparison of the physical-chemical characteristics and catalytic performance of SAPO-11 synthesized in different media. Three SAPO-11 molecular sieves were synthesized in conventional aqueous medium, H₂O-cetyltrimethylammomium bromide (CTAB)-n-hexanol two-liquid phase medium and HF medium, respectively, and were characterized by X-ray diffraction, scanning electron microscopy, N₂ adsorption–desorption at low temperature, pyridine adsorbed IR spectroscopy, and NH₃-TPD. The differences in acid site density and acid strength of these samples are due to the variety of the mechanisms of isomorphous substitution of silicon into the aluminophosphate framework. Catalytic performance of the bifunctional Pt/SAPO-11 catalysts and the product distribution of n-hexadecane hydroisomerization over the catalyst were studied. The relation of the acidity and the catalytic performance of n-hexadecane hydroisomerization have been investigated.