ARTICLE pubs.acs.org/JPCC
Preparation and Gas Adsorption Characteristics of Zeolite MFI Crystals with Organic-Functionalized Interiors Mohamad H. Kassaee, David S. Sholl, and Sankar Nair* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, United States ABSTRACT: We show that the selective gas adsorption properties of pure-silica MFI zeolite crystals are significantly altered by modifying their internal pore structure with molecules such as aliphatic alcohols, aromatic alcohols, amino-alcohols, and amines. NMR and TGA/ DSC characterization indicates that the functional groups are chemisorbed to the internal pores, and the organic molecule loading is strongly dependent on the concentration of internal silanol sites. Adsorption isotherms of CO2, CH4, and N2 are measured for the functionalized samples up to 400 kPa. In the low pressure region (110 kPa), the organicmodified materials show gas adsorption behavior significantly different from that of the bare zeolite MFI, ranging from an enhancement in CO2/CH4 sorption selectivity in the aminefunctionalized materials to an enhancement in CH4 adsorption in the aromatic-functionalized material. The CO2/N2 sorption selectivity decreases for all the modified materials compared to bare zeolite MFI. This investigation of organic-modified MFI zeolites points toward the tuning of adsorption and molecular sieving behavior of zeolites through internal modification of the pores.
’ INTRODUCTION Zeolites, having nanopores less than 2 nm in diameter, are widely used in adsorption processes and shape-selective catalysis, as well as in emerging applications such as molecular sieving membranes, sensors, and low-k dielectric materials.18 By incorporating covalently bound (as opposed to physisorbed) organic groups within the micropores, these highly ordered materials can potentially be converted to organicinorganic hybrids for a diverse range of new applications. There are three main routes15 for functionalizing mesoporous materials and zeolites: (i) direct synthesis via solgel or hydrothermal processes involving co-condensation of organotrialkoxysilanes R-Si(OR0 )3 or organochlorosilanes R-SiCl3 with tetraalkoxysilanes (Si-(OR)4) that are the primary silica source for mesoporous material formation, (ii) postsynthesis modification via grafting the material with silane coupling agents such as NH-(SiR)2, ClSiR3, or RO-Si-R0 , and (iii) postsynthesis reaction with organic molecules. The first two routes present difficulties in their application to zeolite materials. Direct co-condensation has been shown to lead to organic-functionalized zeolites only in rare cases, whereas the grafting of organic groups to the internal surfaces of zeolites using silane coupling agents is impeded by the small pore size of the zeolite. The third route of direct reaction of molecules such as alcohols, amines, and aromatics with zeolite surfaces offers a variety of possibilities for functionalization. Several reports3,4 investigate the esterification reaction of alcohols on silica particles that converts their hydrophilic external surfaces into hydrophobic surfaces. Pure-silica zeolites such as MFI are known to usually contain a number of silanol defects within their internal pore structure. Organic-modification of r 2011 American Chemical Society
these defects may lead to hybrid materials with new molecular recognition properties, but little is currently known regarding the preparation and characterization of such materials. Cheng et al.3 reported the internal surface functionalization of pure silica MFI particles by covalent condensation of the silanol defects with aliphatic alcohols such as 1-butanol and 1-hexanol. The observed alcohol loading was strongly correlated with the concentration of internal silanol defects in the zeolite. Two previous reports5,6 studied the properties of high-silica MFI modified with methylamine (MA). Based on FT-IR and NMR characterization, the formation of SiNSi linkages in the framework was proposed. The MA-modified MFI zeolite showed basic properties. Guo et al.5 investigated the modification of high-silica MFI with various alkyl amines. The basicity of these materials, as measured by CO2-TPD, is 12 orders of magnitude stronger than that of the initial high-silica zeolite. It has also been suggested that the N-containing hybrid zeolites may be useful in base-catalysis applications such as partial oxidation,7 halogen elimination,8 and especially Knoevenagel condensation.9 Functionalization of internal zeolite pores with aromatics is also attractive because they can be further modified to yield a variety of chemically active sites.10,11 The objective of the present work is to ascertain whether significant differences can be found in the interactions of gas molecules with organic-modified MFI in relation to bare MFI. We modify the internal pore structure of MFI with systematically Received: July 17, 2011 Revised: September 6, 2011 Published: September 06, 2011 19640
dx.doi.org/10.1021/jp206822b | J. Phys. Chem. C 2011, 115, 19640–19646
The Journal of Physical Chemistry C
ARTICLE
Scheme 1. Reactions of an MFI Silanol Defect with Organic Molecules Containing OH (a) and NH2 Groups (b)
chosen organic molecules in order to understand the nature of the hostguest bonding and thermal stability of different functional groups and then examine the gas adsorption characteristics of these functionalized materials. Specifically, MFI is functionalized with four types of organic molecules: aliphatic alcohols (1-butanol, 1-hexanol), aromatic alcohols (benzenemethanol), aminoalcohols (3-amino-1-propanol), and amines (1-propanamine, 1,3propanediamine). Scheme 1 shows the hypothesized schemes for the reactions of organic molecules containing OH and NH2 groups with a silanol defect in the zeolite MFI structure. Molecules containing OH groups condense with the silanol defect to form a covalent ROSi linkage. Molecules containing NH2 may form two possible products: a hydrogen-bonded complex or a covalent SiNR bond.5,6 Although the exact nature of the reaction product is still controversial, it is clear that a strongly bound (chemisorbed) species is formed. For molecules containing both NH2 and OH groups, a mixture of the above reaction products is expected. These functional groups provide the potential for a variety of polar and hydrophobic interactions with gas molecules. The structure of these hybrid zeolite-organic materials is studied in detail by a number of characterization techniques. We then report CO2, CH4, and N2 gas adsorption isotherms of these materials, which show a clearly different behavior from that of the bare zeolite MFI framework. These results are explained in terms of changes in the interactions between the adsorbate and the zeolite framework, as created by the presence of the modifying organic groups.
’ EXPERIMENTAL METHODS Pure Silica MFI Crystal Synthesis. To minimize external surface contributions, we synthesized 10 μm crystals of puresilica MFI via the procedure of Agger et al.12 For 10 μm particles, the external surface area is less than 1% of the total surface area13,14 and would contribute a negligible organic loading (
