Introduction
Long chain linear alkylbenzene (LAB) is one of critical chemicals since its sulphonated products are the raw materials for industrial and domestical detergents.1Commercially, the LAB is produced through the alkylation between benzene with α-olefins (C8-C16), which is catalyzed under the HF and AlCl3system.2 Notability, the position of phenyl group in long chain α-olefin is crucial to biodegradability and solubility, among various LAB isomers, the 2-LAB is the most biodegradable and possesses the best solubility.3 Therefore, it’s highly desired to develop an efficiency catalysis technology to improve the selectivity of 2-LAB isomer. However, the traditional HF and AlCl3 catalysis technology for the production of LAB brings some severe issues: 1) these homogeneous catalysts can generate a large number of waste acids during the producing process, they also cause serious corrosion towards reactor, and the separation with the reactants and products is a tough energy-consuming process.4 In addition, the HF requires special handling and equipment, which further increase cost and risk; 2) HF and AlCl3 catalysts exhibit low 2-LAB selectivity with ~15% and 30%,5 respectively. This is far from the target that develops an environmental-friendly, noncorrosive, reusable and green alkylation technology with desired 2-LAB selectivity.
To overcome these problems, a large number of clean technology has been investigated based on the solid acid catalysts such as zeolites,6,7ion exchange resin, 8metal oxides,2clay9 and heteropoly acid.10 Among them, the UOP company firstly realized the production of LAB in the industrial scale based on the Detal technology by employing HF or NH4F modified SiO2-Al2O3 as the solid catalyst,11 which open a window for the development of green alkylation technology between benzene with long chain α-olefins. Currently, the production of LAB mainly depends on the HF, AlCl3 and Detal technology, and the corresponding market share is ~75 %, 10 % and 15 %, respectively. Unsatisfactorily, the selectivity of 2-LAB is only increased to 25 % through Detal technology.12 Besides, the acid strength of HF or NH4F modified SiO2-Al2O3 is not enough to catalyze the alkylation of benzene with long chain α-olefins, and the reaction must be operated under the high temperature and pressure, resulting in high cost compared with conventional HF technology. Therefore, it’s still highly expected to obtain an efficient green technology for the production of 2-LAB.
Zeolites is a class of widely used solid catalysts with tunable acidity and pore size, which is an ideal candidate for alkylation between benzene with long chain linear α-olefins. In the past decades, great efforts have been made to search suitable zeolites for 2-LAB synthesis.13 The major issues that prevent the successful industrialization of zeolite catalysts for LAB process are the non-ideal distribution of LAB isomers and deactivation of zeolites.6 On the one hand, the activity of zeolites is from acid sites that can catalyze alkylation between benzene and α-olefins. Thus, zeolites must expose more accessible acid sites in order to improve activity, the effective strategy for improving accessible acid sites in zeolite framework is fabrication of mesopore zeolites with open hierarchical channels. The mesopores not only improve the accessibility of acid sites, but also enhance the diffusion of coke precursor and increase the life time of catalysts. On the other hand, the shape-selective catalysis for zeolites is mainly based on the size or shape in sterically confined environments.14 If the mesopores are introduced into the framework or special fabrication of layer zeolites with ultra-thin thickness along one crystal axis, the shape-selective effect of zeolites is lineally decreased. Therefore, it’s still a huge challenge to fabricate suitable zeolite catalysts for the production of 2-LAB.
In present research, the structure of MWW zeolite are optimized to solve the above-mentioned issues. Dual template strategy with conventional hexamethyleneimine (HMI) structure directing agent and long chain quaternary ammonium surfactant is employed to delaminate the MWW zeolites, which expect to expose the external acid sites and improve the accessibility of acid sites. Our finds confirm that external surface Brønsted acid sites can be systematically tailored by tuning the hydrophobic chain length of surfactant. Moreover, the mono-layer delaminated MWW zeolite is obtained by increasing the ratio between quaternary ammonium surfactant and HMI. Computational results reveals that the external isolated 12-ring hemicavities located on the upper and lower surface of the MWW lamella can stabilize the 2-LAB due to the most appropriate sterical configuration and highest binding energy with 12-ring hemicavities, which is benefit to the formation of 2-LAB. Our study shows that a regularly spaced MWW zeolite nanosheets system can greatly improve the activity of catalysts in the alkylation between benzene with 1-dodecene without sacrificing the selectivity of 2-LAB. In addition, the catalytic performance of MWW zeolite nanosheets are further maximized by altering the distribution of Al species on the exterior surfaces and inhibiting the stacking of MWW zeolite layers, the optimized MWW zeolite layers also show a superior stability with robust selectivity of 2-LAB, which is highly desired and expected as an ideal solid catalyst for the industrial production of 2-LAB.