Z. Gabelica

Identification of different tetrapropylammonium cations occluded in ZSM-5 zeolite by combined thermal analysis (t.g.-d.t.a.) and 13C-n.m.r. spectroscopy

The combined use of thermal analysis (t.g.-d.t.a.) and 13 C-n.m.r. spectroscopy allows one to identify four different structural environments for TPA + ions occluded in the ZSM-5 channels. A first low-temperature (ca. 380°) d.t.a. peak is assigned to less strongly held or outer shell “strained” TPA + ions (type I), while a large intermediate temperature (ca. 425°C) peak C1 is attributed to inner-strained TPA + ions (type II). Both TPA + species neutralize the SiO − defect groups. The high-temperature (ca. 475°C) peak is attributed to two different TPA + ions: those neutralizing the framework (Si-O-Al) − negative centers (type III) (for high Al content samples), and those in the relaxed form that also neutralize SiO − defect groups (type IV) (for both high- and low-Al-content samples). Types I–III all show the same 13 C-n.m.r. spectra (splitting of the CH 3 -n.m.r. line), while type IV exhibits only one n.m.r. line for the CH 3 - groups.

Aluminium distribution and cation location in various M-ZSM-5-type zeolites (M Li, Na, K, Rb, Cs, NH4)

Combining PIGE, EDX, AA, and thermal analyses with solid-state 29Si-n.m.r spectroscopy allowed us to determine the amounts of SiOR and (SiOAI)−R+ groups (R H, M, TPA). The amount of SiOR increases with decreasing aluminium content. At the external surface of the crystallites, an Al-rich phase is deposited, which is revealed to be a zeolitic phase rather than a MAIO2 species. The d.t.a. peak at ca. 360°C is attributed to external shell TPA species. Finally, the partial solvation of the cations is shown by both 7Li- and 23Na-n.m.r. results.

Structure and stability of tetrapropyl-ammonium (TPA) ions in various alkali—aluminosilicate hydrogel precursors to crystalline (M)ZSM—5 zeolites

Abstract The stability of TPA species interacting with various (Al,Si) hydrogel environments in the presence of alkali cations has been evaluated by thermal analysis (combined t.g.-d.t.a.-d.t.g.). Three different TPA—gel associations have been characterized by d.t.a. endotherms : crystalline (TPAX) n clusters (X = OH − ,Br − ) which are also detected by XRD, monomeric (TPAX) species in direct association with the (Al,Si) gel, and labile entities, [TPA + …(H 2 O) x ] bulky water clathrates, that interact very weakly with the remaining negative charges available after neutralization of the gel by small (Li,Na,K) alkali cations. The formation, relative concentration and stability of these species essentially depend on the relative amounts of Al 2 O 3 and M 2 O present in the gel, as well as on the nature (structure forming/breaking) of the alkali cations. Monomeric as well as water clathrated species lead to crystalline (M)ZSM—5 upon heating.

Parameters affecting the optimal synthesis of zeolite ZSM-20

Abstract Careful control of various synthesis parameters such as the composition of the precursor hydrogel, temperature and time, allowed the preparation, in optimum yield, of zeolite ZSM-20 from a hydrogel that usually yielded the more stable zeolite Beta. From a detailed physio-chemical characterization of both zeolites, essential differences were found between their framework structures and compositions. From a kinetic study of the formation of zeolite ZSM-20 under optimal conditions, we propose a mechanism which involves a liquid phase ion transportation.

About coke deposition on zeolite HY: A129Xe-NMR study

Abstract NMR investigations of 129 Xe sorbed on coke-fouled zeolites provide quantitative information on the distribution of carbonaceous residues during coking and after decoking.

GEL Composition Versus Organic Directing Agent Effects in the Synthesis of ZSM-39, ZSM-48 and ZSM-50 Zeolites

Abstract Zeolite ZSM-48 prepared from a hydrogel containing an alkylamine admixed with TMA + ions incorporates these organic species, so that the total filling of the porous volume is achieved. However, when zeolite ZSM-39 crystallizes from a similar hydrogel (also containing TMA + ions, and with or without alkylamine and aluminium ions), part of the TMA + ions are dequaternated to trimethylamine and trimethylammonium species, which are also occluded into the large cavities of the ZSM-39 crystals. Hydrogels including hexamethonium (HM ++ ) ions, with or without ammonium species, lead to the formation of zeolites ZSM-48 and ZSM-50, depending on the initial aluminium content. In all cases, the amount of organic species occluded (either only HM ++ ions, or HM ++ together with hexyltrimethylammonium and trimethylammonium ions which result from the decomposition of the HM ++ ions) is governed by the filling of the zeolitic free pore volume.

Role of Alkali and Tetrapropylammonium Cations in (M)ZSM-5 Hydrogel Precursors

The influence of alkali and tetrapropylammonium (TPA + ) cations on the organization of hydrogel precursors to ZSM-5 zeolite is examined by multinuclear high power solid state nuclear magnetic resonance techniques. Emphasis is put on the different states of the TPA + ions and on the interaction between the alkali cations (M + ) and the negative charges in the gel.

Physicochemical characterization of zeolite ZSM-20

Abstract Zeolite ZSM-20 is a silica-rich zeolite, possessing a faujasite-like character but having a hexagonal symmetry unit cell. It is synthesized in the presence of tetraethylammonium (TEA) cations as organic directing agents. Those act as counterions, in addition to Na+ cations, and are eventually occluded in the zeolite large cages where the electric field gradient is small.

A 13C - and 129Xe-NMR Study of the Role of Tetraalkylammonium Cations in the Synthesis of High-Silica Zeolites

The high silica gel is well organized around the tetraalkylammonium (TAA) cations during prenucleation stage. By combined 13C- and 129Xe- NMR, two TAA+ states, either hydrated or dehydrated, and three different TAA-gel associations can clearly be distinguished: n n1. n ndispersed monomelic or dimeric (either hydrated or dehydrated) TAA+ species (1–2 cations per cavity, cavity diameter ~11–16 A) n n n n n2. n nless dispersed TAA+ ions (4–8 ions per cavity, cavity diameter ~17–23 A) n n n n n3. n nlarge aggregates (TAAX)n (X=OH, Br) (n gt; 15 per cavity, cavity diameter ~30–40 A)