a(n) = 6*binomial(4*n-1,n-1)/(4*n-1).

Terms

a(0) =2a(1) =6a(2) =30a(3) =182a(4) =1224a(5) =8778a(6) =65780a(7) =508950a(8) =4034712a(9) =32602328a(10) =267535086a(11) =2223463866a(12) =18676869400a(13) =158310871740