Brønsted acidity of amorphous silica-aluminas for hydrocracking of Fischer-Tropsch wax into diesel fractions

https://doi.org/10.1016/j.apcata.2021.118439Get rights and content

Highlights

  • ASAs were synthesized from a precursor of Beta.

  • In-situ TMPy-Py sorption FTIR was innovatively used to quantify the Brønsted acidity in micropores.

  • The strong Brønsted acid sites in micropores lowered the diesel selectivity.

  • 78.7% diesel selectivity at 48.8% conversion was achieved in FT wax hydrocracking.

Abstract

A series of amorphous silica-aluminas were hydrothermally synthesized in the presence of tetraethylammonium hydroxide. Their physicochemical properties were well characterized, and particularly, a facile method of preferentially adsorbing bulky 2, 4, 6-collidine followed by adsorbing pyridine for FTIR was innovatively used to quantitively distinguish the Brønsted acid sites in micropores and on external surfaces. All protonic samples were transformed into hydrocracking catalysts by loading with 0.5 wt% platinum; and the relationship between Brønsted acidity of supports and the catalytic performance was studied by hydrocracking of Fischer-Tropsch wax. The results illustrated that the medium strong Brønsted acid sites located on the external surface play important role on the activity. And the lower the strong Brønsted acid density in micropores, the higher the diesel selectivity. Among the catalysts tested, Pt/SA-10 catalyst exhibited high selectivity to diesel (78.7%) at about 48.8% conversion under reaction conditions close to industrial ones.

Introduction

As one of the alternatives to petroleum-based fuel, Fischer-Tropsch (FT) liquid fuel has already been practically produced from natural gas, coal and biomass [1], [2], [3]. In the large-scale industrial application, however, FT wax can account for more than 50% of the total iron-based FT synthesis products and even more in cobalt-based process [4], [5], [6]. Greatly different from the traditional refinery hydrocracker feedstock from crude oil feed, FT wax is highly paraffinic and ultra-low in aromatic content, which makes it ideal to produce high-quality transportation diesel fraction through hydrocracking [6], [7]. Moreover, these FT wax has a very wide carbon number distribution, which typically ranges from C20 to C80, due to the chain growth mechanism in FT reaction process [6], [8]. And the longer carbon-chain FT wax compounds crack more easily and inevitably undergo secondary cracking, which reduces the selectivity for diesel [9], [10]. Meanwhile, the different nature of FT wax makes it unlikely to directly apply traditional refinery hydrocracking catalysts [6], [11]. Considering this, efforts particularly on the study and optimization of hydrocracking catalysts tailored for FT wax feed are indispensable, especially on the study of structure-activity relationship.

Hydrocracking catalysts are commonly bifunctional and composed of a metallic compound dispersed over an acidic support [7], [12], [13]. The metallic compound can be either noble-metals, such as Pt, Pd, or transition metals, such as Ni, Mo and W, usually in a sulfide state, which provides the hydro/dehydrogenation (HD/DHD) function [14], [15], [16]. As a synergy match, the acidic support component offers the isomerization and cracking functions, and can strongly influence the catalytic hydrocracking performance [17], [18]. Zeolite crystals and amorphous silica-aluminas (ASAs) are both known to be effective acid supports, while to convert the bulky and easily-cracking FT wax, ASAs become more positive in terms of facilitating molecular diffusion as well as providing moderate acid sites [19], [20], [21], [22].

Extensive discussion and research have been done on ASAs-based hydrocracking catalysts for both scientific and application purposes. Earlier, Corma et al. reported a bifunctional catalyst based on a new mesoporous silica-alumina (MSA) obtained from a precursor of ZSM-5, which exhibited higher selectivity towards isomerization than USY zeolites in the hydroconversion of n-heptane [23]. It was found that the lower Brønsted acid strength of Pt/MSA contributed into limited secondary reactions as compared to the Pt/USY catalyst, meanwhile the presence of uniform mesopores promoted the diffusion to gas phase and lowered the probability to undergo further cracking. Despite of the size differences between their model compound n-heptane and FT wax, the above work suggested that mild acidity and good diffusion properties could effectively inhibit further cracking. In terms of the bulkier FT wax compounds, the pore diffusion resistance, in another word, the mass transfer and the reachability to the acid sites would be even more substantial [12]. Kim et al. carried out the hydrocracking of wax (C21-C36 n-paraffins) to middle distillates (C10-C20) over Pt@ASAs catalysts, and reported that the Brønsted acid sites density could be tuned by the Al content, and the highest yield of middle distillates was received over Pt/Si0.7Al0.3Ox [24]. Another study by Lee et al. focused on the effect of the acidic properties of ASAs in the hydrocracking of paraffin wax [21], [25]. It pointed out that the conversion of n-paraffin wax increased with the total acid sites density, while the yield of diesel-range products exhibited a volcano-shaped dependence instead. Among the catalysts tested, Pd/SA-40 catalyst with moderate acidity showed the highest yield for middle distillates. Although many studies have been carried out on the synthesis and application of ASAs-based hydrocracking catalysts, few studies have systematically examined the structure-activity relationship, especially from the prospect of the combined effects of location, strength and density of the acid sites on ASAs supports [20], [26], [27].

A proper characterization on the acidity details is the precondition to this study. Infrared spectroscopy of substituted alkylpyridines sorption is currently the most common method, which could provide the quantitive information of the acid sites type, strength and numbers of a solid acid material [28], [29]. It has to be aware that the acid sites density quantified according to Lambert-Beer Law, varies depending on the integrated molar extinction cofficients (IMEC) of the alkylpyridine [30], [31]. And the literature data on the IMEC of the FTIR bands vary greatly from each other even by a factor of about four, and the origin of that is not clear but may lie in the experimental procedures [30], [31], [32]. Therefore, it will be beneficial to weaken the influence of IMCA as much as possible to improve the accuracy and effectiveness of acid characterization results. In addition, to collect the aimed information, the acid sites have to be accessible to the particular probe molecule used. This offers an opportunity to measure the acidity on different position by choosing alkylpyridines with different dynamic sizes (for example, pyridine (Py) with 0.57 nm, 2, 6-lutidine (DMPy) with 0.67 nm and 2, 4, 6-collidine (TMPy) with 0.74 nm) [28], [29]. Liu et al. characterized the acidity of H-ZSM-22 by applying Py-FTIR and DMPy-FTIR, respectively, to obtain the total Brønsted acid sites density and that at outside of micropores, and then discussed the structure-activity relationship for long-chain n-alkane isomerization [33]. Although the above-mentioned methods managed to quantify Brønsted acid sites, the variation of the Brønsted acidity inside the micropore as well as the comparison of that inside and outside of the micropore, was not available as the differences in quantivive unit for Py-FTIR (μmol Py / g) and DMPy-FTIR (μmol DMPy / g) measurement [29], [34]. This limits the understanding on the acidic nature and the acidity-performance relationship.

In the present work, a series of ASAs with different structure, acidity were successfully synthesized in the presence of organic compounds prior to Beta zeolite formation, and characterized combining multiple techniques, such as XRD, FTIR, TEM, BET, Py-FTIR and NMR. Particularly, to quantitively distinguish the Brønsted acid sites in micropores and on external surfaces respectively, a facile method was innovatively used by preferentially adsorbing bulky TMPy and then adsorbing small molecule Py in FTIR characterization. The relationship between structure / acidity of these as-synthesized ASAs and catalytic performance was built over Pt-loaded ASAs catalysts for FT wax hydrocracking reaction. Moreover, the effect of the acidic nature of ASAs supports on the catalyst activity and the selectivity for diesel fractions (C10-C22) were discussed, which offered a good opportunity for better understanding protonic materials and for more rationally developing hydrocracking catalysts tailored to FT wax feed.

Section snippets

Reagents

The following chemicals were used as received without further purification: tetraethylammonium hydroxide (TEAOH) (25 wt% Runjin Chemical Co., Ltd.), silica sol (30 wt% Qingdao Haiyang Chemical Co., Ltd.), sodium aluminate (Sinopharm Chemical Reagent Co., Ltd.), sodium hydroxide (NaOH) (Xilong Chemical Co., Ltd.), ammonium chloride (NH4Cl), (Tianjin Guangfu Technology Development Co., Ltd.), tetraammineplatinum chloride hydrate (Pt(NH3)4Cl2•XH2O) (Aladdin Chemical Reagent Co., Ltd.).

Catalysts preparation

Standard

Structure and morphology

Fig. 1a displays powder XRD patterns of the products collected after variable hydrothermal synthesis time. Samples SA-5, SA-10, and SA-15 obtained by hydrothermal treatment for 5 h, 10 h, and 15 h respectively, show only a broad hump at 2θ = 15 o~ 30 o, an amorphous characteristic [36], [37]. And upon heating, this broad hump slightly shifts toward higher angles (Fig. 1a inset; dotted arrows), probably due to the formation of smaller ring structures of aluminosilicates [36]. The induction

Conclusions

A series of ASAs with different physicochemical properties were synthesized from TEAOH containing hydrothermal system before zeolite Beta crystallized. A facile method of preferentially adsorbing bulky TMPy followed by adsorbing Py for FTIR, was innovatively used to quantitively distinguish the Brønsted acid sites in micropores and on external surfaces,which effectively avoided the error caused by different IMEC of different probe molecules, and offered a better opportunity to characterize all

CRediT authorship contribution statement

Tao Li: Investigation, Validation and Writing – original draft. Zhichao Tao: Funding acquisition, Conceptualization. Caixia Hu: Resources, Formal analysis. Chunli Zhao: Formal analysis. Fengjiao Yi: Formal analysis. Guoyan Zhao: Formal analysis, Resources. Ling Zhang: Conceptualization, Writing – review & editing. Yong Yang: Writing – review & editing, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Beijing Science and Technology Commission-urban area two major urgent tasks science and technology (Z181100009818003), Research and Development of Ultra-clean Gas-Coal Diesel Technology Package from Coal Indirect Liquefaction Intermediate Products from Ordos Bureau of Science and Technology, and “The key technology for producing high quality aviation coal components from Fischer-Tropsch light oil”, the National Key Research and Development Program of China, Grant No.

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